There are a ton of planets in GFFA that can support life. There are tons of exoplanets in our own Milky Way Galaxy have no sign of life. So is the Star Wars galaxy plausible with its so many planets full of sentient life, or is it truly a work of implausible fiction?
Some astronomers now suspect planets around nearly every single star. There are 300 billion stars in the Milky Way. We know of very few exoplanets, and we tend to see only the larger ones. Quite possibly, those same star systems have smaller, more amenable planets.
What we know so far is such a small sample size of the whole, that I don't think it can in any way be representative of this galaxy.
Now, it's unclear whether sentient life is typical on a life-bearing world. So perhaps they exaggerate a bit on that front. But if they have 1 million species in that science fiction universe, it would still only be 1 per 300,000 stars. Additionally, an intragalactic economy might go far to preserve sentient species that would go extinct on their own... thereby boosting the numbers you'd see. This is speculative, but in a galaxy where faster-than-light travel is possible and there are thousands of sentient species zipping around, might they not have decided to take action to prevent the extinction of Neanderthals on our own world?
Of all the highly implausible things that are depicted in Star Wars, I do not count the depiction of numerous sentient species to be one of them.
When you say "tons of exoplanets in our own Milky Way Galaxy have no sign of life", you are referring to the state of knowledge in astrophysics today. This, as any examination of the history of science shows, can change.
A lot of what we accept as "facts" from science are merely "the closest assessments available today". No one on Earth has been on those planets and near those stars to determine if there is life, all we can do is suggest models that take our own experience - our own planet with our own species - and project that onto the the galaxy. "That planet is close to its sun, and if that had happened here, it would probably be too hot for creatures like us". There are a lot of assumptions made here - assumptions freely admitted by the scientists themselves, though they tend to get blurred when reported - that these "no sign of life" planets are only blacklisted for very specific signs of life, in very specific conditions.
So the explosion of life-supporting planets in the GFFA isn't really implausible, as long as you accept the basic assumption that what we know today isn't the whole truth. And that's an assumption that you have to make for science fiction in general. And, to be honest, for science, too.
Yes, it's plausible, given our current knowledge.
If we lived in the Star Wars galaxy, assuming no direct contact with other civilizations, it would look much like this one given our current technology. We'd have started being able to detect exoplanets only recently, but only ones that are very large and/or very close to their stars -- just as we're doing here and now.
We might know about the gas giant Yavin if it were close enough. We wouldn't yet be able to detect its habitable moon Yavin 4.
Just recently, we've already started detecting planets with just a few times the mass of Earth within their stars' habitable zones. But we just don't know whether they actually support life, because we haven't yet been able to examine them closely enough. Kepler-22b could be swarming with Ewoks for all we know.
We don't actually know that there are other Earth-like planets in our own galaxy, but given the distribution of planet sizes and orbits in our own Solar System, and of the subset of exoplanets we've been able to detect, it's very likely that there are plenty of them. It's entirely plausible that our own galaxy is as rich in habitable planets as the Star Wars galaxy.
There is at least one one visible difference. The Star Wars galaxy is home to a galaxy-wide civilization with easy faster-than-light travel (an Empire or a Republic, depending on when you ask). Most civilizations would have been contacted by now, and probably annexed. The Empire certainly had no qualms about setting up bases on primitive inhabited worlds, such as the Forest Moon of Endor.
But given that we have just a single example of a galaxy with no apparent galaxy-wide civilization, that doesn't make the existence of such a civilization in another galaxy implausible.
There are tons of exoplanets in our own Milky Way Galaxy have no sign of life.
Yes, but about 1% of them do exist in what we refer to as the "Goldilocks Zone", where liquid water can exist. Just because 99% or more of all planets couldn't possibly harbour life (among those that we've found, most of them are so hot that you could melt iron and rock on the surface - we've found planets hot enough to evaporate sand - and thus rain liquid glass), doesn't mean that life is not abundant in our galaxy. You just have to remember that the numbers are literally astronomical. There are hundreds of billions of stars in our galaxy. If 1% of them have life, that means there's billions of habitable worlds out there. If 1% of those have life more advanced than bacteria, that narrows it down to "only" hundreds of millions of worlds with plants and insects. If 1% of those have intelligent life capable of forming civilizations, that still means there are millions of planets with such. And we're now talking about a ratio of 0.0001%! For space-faring civilizations to number in the hundreds or even thousands in an area as large as a galaxy, the chances of that don't have to be very high at all, even shrinking to infinitesimally small.
If I had to pick one least implausible thing about the star wars universe, it might be the proliferation of different sorts of life in it. Some of the forms might be less plausible than others - for example, the giant space worm from Empire seems a bit of a stretch - but the sheer number is exactly what you'd expect given the vast number of opportunities for life to arise, and the amazing capacity for life to adapt to wherever it finds itself. On the other hand, among the more implausible elements of the star wars universe is the fact that these life forms meet and interact. So depending on whether you're interested in the raw number of forms of life or the number of different sorts of aliens in the Cantina, my answer is either "yes, it's quite plausible" or "no, it's a complete fantasy".
We've only been to one of the hundred(s) of billions of star systems in our own galaxy, so I don't think we're qualified to say "how plausible" a galaxy with many species in it is. Also, as far as I know, we don't know the history of race proliferation in the Star Wars galaxy. The fact that many of them are very human-like or at least humanoid, may suggest that many races may have originated on one planet long, long ago, and spread around to different planets long, long before the events shown in the films.
The most implausible thing about the life forms in the films, to me, is the depiction of the water creatures in Episode I, when Obi-Wan and Qui Gon are taking a submarine on Naboo, to meet Jar Jar's New Orleans-esque underwater people (...which also re-destroyed my credence, for other reasons...). During the submarine ride, we are shown a series of increasingly-large "fish" swallowing each other, while the Jedi *cough*nonsensically*cough* "impress" us with their lack of concern. That scene was a peak of implausibility for me, because unless they were just there for an incredibly unlikely and infrequent event, I can't imagine that any environment could sustain that phenomenal rate of biological destruction, because of the time surely required to generate the larger fish, shown being destroyed in seconds. All I can think (besides that it was done in a lame attempt to impress audiences assumed to be dumb and in need of excessive spectacles) that might make that plausible, would be if those creatures don't actually digest each other, but instead are about to vomit each other out alive, or they do some bizarre reformation after swallowing each other.