Were the Jedi ever shown to be the amazing heroes that people expected Luke Skywalker to be? In the prequels, they were shown to be too set in their ways and bureaucratic. After the purge, Obi-Wan and Yoda essentially abandoned the Jedi Order. Just because all the other trained Jedi were killed doesn't mean potentials ceased to exist, yet they both became hermits. And for all their talk about "too old to train" with Anakin, why did Obi-Wan wait until Luke came to him--in his early 20s--to train him? Why not start his training as a toddler since he lived just a couple sand dunes away? Had he ever planned on training him? If Leia hadn't sent that message with Artoo, or if Luke had never found Artoo, then Luke would probably never have been trained and the Jedis would be gone. But maybe it was a good idea to let the Jedi die out, considering the Master/Apprentice lineage of the surviving Jedi. Obi-Wan's first apprentice was Darth Vader and Obi-Wan's master (Qui Gon) was the apprentice of Dooku, who had been trained by Yoda. Given that, Luke 1) having a fear of losing control and 2) giving up, because that's what Obi-Wan and Yoda did when the going got tough, makes sense.
I've been thinking that The Last Jedi hints that they trained the wrong Skywalker twin. Luke was an accidental hero and kind of a flake. Leia chose her role in the rebellion and has pushed on through all the shit life has thrown at her. Her entire planet and family/friends got blown up, and she kept going. She finds out the evil dude who tortured her (and also involved in blowing up her planet) is her bio-dad, and she keeps going. Her husband and brother ran off on her, and she kept going. Her husband was murdered by their son, and she kept going. Her self-rescue via the Force shows that she had the potential. If Leia had been trained, there would probably be a smoothly running Jedi Academy, with new Jedi to protect the galaxy. And if someone burned it down on her, rather than running away to become a hermit she would have built another one and kept going.