I can't help your misinterpretation of events. You've decided this and we seemingly can't sway you with facts about fiction.
Even if we accept 15 years, which doesn't seem to match what was in the opening crawl, it still makes the rebellion a failure. A short break in tyranny still doesn't change that they accomplished very little and the original trilogy was a failure thanks to Abrams.
Which was established in the film. Luke died doing the most Jedi thing imaginable. He used the Force for knowledge and defense, not for attack.
He essentially played a video game and died.
And yet, we've never seen a Force Skype call or lightning like Yoda did. So, there is clearly some expansion of the lore based upon concepts Luke had, as well as in the books. So, there is a huge amount of assumption going on regarding Force ghosts.
And a huge amount of room to create more things.
My feeling is that, ‘Luke as Force Ghost’ may in fact be more powerful and utterly different to that of what ‘Force Ghostdom’ has previously entailed, based on PT and OT.
Which might be interesting if they go into the lore of Force ghosts and start explaining things about them. I doubt that happens, but it would be fascinating to see Luke disappear again, and then see what happens to him. But if all he does is reappear and do cameo things, it's really nothing special.
...So? I don't see the problem. He still died doing something heroic.
Either way It was said earlier in the film that using that kind of power could kill someone.
No he didn't. He died playing a video game, projecting a hologram. His death added nothing to the plot. If he lives, nothing changes.