In the series finale of Star Wars Rebels,
we see that Ahsoka Tano is alive after the events of the Original Trilogy, and it is conveyed to the audience via Sabine's epilogue voiceover that Ezra is still alive.
However, the entire driving force of the Original Trilogy is that Luke, Obi-Wan, and Yoda are the only Jedi left, and it is up to Luke to overthrow the Empire and rebuild the Jedi (to pass on what he has learned, as it were).
This exchange in Empire:
KENOBI: That boy is our last hope.
YODA: No, there is another.
Obviously, the "other" refers to Leia, Luke's sister who is revealed to be Force-sensitive at the end of Empire. So Luke is the last Jedi, with Leia a possible replacement if Luke fails.
This is also communicated to the audience when Yoda dies in Return of the Jedi:
YODA: Luke, when gone am I, the last of the Jedi will you be. Luke, the Force runs strong in your family. Pass on what you have learned, Luke... There is another Skywalker.
We also know that
Yoda knows who Ahsoka Tano and Ezra Bridger are—and that they are alive shortly before the events of the Original Trilogy—as he communicates with them in the Rebels series.
Does this mean that Yoda and Kenobi
presumed Bridger and Tano to be dead during the events of the Original Trilogy?
If Yoda knew they were alive, he wouldn't have told Luke that he was the last Jedi. And I presume that he is (a) not lying to Luke and (b) is Force-sensitive enough to know if these characters were still alive.
I assume that Dave Filoni would not write an ending to a series that took a large amount of the emotional weight out of the Original Trilogy—but I felt like this ending did just that. If Ezra Bridger and Ahsoka Tano are alive, then Luke's importance in the Original Trilogy feels diminished. I am well-aware that an out-of-universe explanation is that this ending is a device to simultaneously explain why these characters were not present in the Original Trilogy while keeping them alive for a future project; however, I am wondering if there is a more satisfying, less-cynical, and in-universe answer.