Where is it specifically stated that the T'au could be easily defeated by every other race in 40k?

Question

There is an almost universally accepted notion in the Warhammer 40k community that the extermination of the T'au would be a trivial matter for any of the other races so much so that not one believes it's worth the effort or they're too busy fighting amongst each other to bother. Which is "said" to be the only explanation for their continued existence.

I'm not here to challenge that. As they are they don't pose a grave threat to the imperium as a whole. The T'au are the youngest, most naive race of significance in the galaxy who know little outside their area of space which is miniscule compared to the Imperium's. And their ability to travel is very limited.

Not a threat, no but is wiping them out really that easy? Could it be a harsh oversimplifcation by the fans just like with Abaddon before they reworked his story in that he was a harmless failure for losing in 12 black crusades?

I'm asking for a specific quote perhaps in the novels or other established source of 40k information like a codex that says it is.

Answer 1

Found in the 8th edition T'au Empire Codex, page 16:

To some of the High Council, the Imperium seemed so powerful that overt aggression might lead to a war that they could not win.

I think that's the clearest statement you can get.

---

One hint could be the Damocles Gulf Crusade, quoted from the Fandom Article.

The Damocles Gulf Crusade, also called the Damocles Crusade, was the first military conflict fought between the Imperium of Man and the rapidly expanding T'au Empire in the Lithesh Sector of the galaxy's Eastern Fringes between 742 and 745.M41.

...

Alarmed by the threat of alien contamination, the Administratum readied a suitable response and almost a century later, the Damocles Crusade smashed into T'au space, destroying several outlying settlements and pushing deep into the T'au Empire.

When the Imperial fleet reached the T'au Sept World of Dal'yth Prime, however, the crusade ground to a bloody stalemate as the formidable numbers and high technology of the T'au and their Kroot allies thwarted every attempt to capture the world or its star system.

So, the notion of the T'au being easily crushed cannot be uphold (at least not to the extend of my knowledge) by a quote from any official source. To the contrary, the T'au have met several times with Space Marines and were able to hold their ground and also stopped a whole crusade fleet, although a small one.

The meme that the T'au could easily be grounded comes from the size difference of the empires. Whereas the T'au Empire consists of some hundred worlds, millions of worlds are in the Empire of Man. However, with the size of the Empire of Man also comes a multitude of dangers from all site that need to be met.

So, only from the size, the Empire of Man could easily crush the T'au Empire. However, the available force to fight against the T'au is only a very small fraction of that. It took the munitorum nearly a century to gather the ressources to attack the T'au and that wasn't enough to overroll them. We therefore have to conclude that it is not as easy to eradicate the smaller T'au empire from the galaxy.

Answer 2

The T'au Empire comprises about 300 planets. Let's go over what might happen with each race attempting to conquer/exterminate the T'au.

The Imperium: They already tried that. It...didn't go over that well. If they were to go and kill the Tau as quickly as possible, they'd nuke T'au (the planet) and Vior'la (one of the planets known for training Fire Caste warriors).

Chaos: Probably the same thing as the Imperium, but they really don't care as much about the Tau as they do about the Imperium.

Craftworld Eldar/Ynnari/Harlequins: It'd probably be a war where losses would be at an all time low, because both races aren't particularly large. In the end, the Eldar would win because they have gods on their side, and almost all of them are psykers. The Tau cannot counter psykers.

Necrons: This one would be fun. Like the Eldar, the Necrons have gods on their side. They use these gods as fuel and as slaves. The Necrons are extremely powerful. They care not for morale, as the lower ranks are little more than automatons. Rather, the Tau would be scared out of their minds as Necrons they thought dead heal from mortal wounds and reassemble. Flayed Ones and Destroyers would further detract from their morale. But what would be worse would be a T'au seeing their comrade disintegrated before their eyes by a gauss flayer. The Necrons would eventually move slowly through the systems that make up the Empire, deploying Destroyers to scour worlds clean. That, or Szarekh would 1v1 Aun'va.

Drukhari: Aside from their gruesome torture methods, they won't stand a chance. The Tau do pack serious firepower.

Tyranids: They'll just eat everything. The Tau would hold out for a bit, and maybe one of the other factions would come to their aid because they hate the space bugs of death more than they do the blue space communists, but it wouldn't go well.

Orks: WAAAAAAAAAAAAAAAAAAAAAAAAGH!!! The Orks would rampage through the Tau systems, caring not for their losses. In fact, they'd get more powerful as Big Meks cannibalise Tau war machines so as to make their own.

Hope this helped!