The reason that works in the Thrawn trilogy is that it takes place almost immediately after RotJ. By the time period covered in TFA, the Empire's been pretty much defeated and they've moved on to a new enemy. TFA gives the impression that almost nothing has happened over the last 30-odd years and, if anything, the Empire/First Order is actually more powerful than it ever was while the Emperor was alive.
Like Ving, I haven't seen the film, but I will eventually and I'm trying to keep an open mind, but a lot of the core concepts seem to be fundamentally flawed.
Sure, a lot of the EU was messy, but most of it was pretty good and built up an insane amount of lore for the universe, while Lucas was pretty happy to say, 'This is like this just because,' and leave it at that 
Finally, if anything should have been made non-canon, they should have started with the prequels 
Then the complaints that they didn't to the Thrawn Trilogy make even less sense. They couldn't do the Thrawn Trilogy exactly as it was written. The characters have aged too much... So, instead, they did a similar story with new, arguably more interesting characters... And never does it leave the impression "nothing" has happened over the last 30 years... It clearly demonstrates that a ton has happened. The fight didn't end with Vader and the Emperor. As is always true in Star Wars, a power struggle began almost immediately. But, I can't get into what HAS happened because I don't want to be rude to the people who haven't seen it (spoilers).
I'd like to know what core concepts you're speaking of, though. Again, spoilers, so maybe PM? Because the core concepts seem to be Thrawn and ANH adjacent... Similar themes with a twist. So... I mean...
Some of it was pretty good. But, it was too expansive and too much of it was bad. Disney clearly didn't want to be tied down to the bad, convoluted ideas presented in them. The small parts of good were sacrificed to free the hands of creators. And that makes the future exciting. Also, they didn't technically destroy them. They made the "Legends" which sort of means they may have happened, may not of, no one really knows, but it isn't part of the canon story (the stuff people know happened 100%). Which means they'll probably still borrow ideas from it. The best ones, anyway.
And finally, I agree. Prequels need to be redone and done correctly. But, it wouldn't surprise me if GL might have put something in the sale agreement that forces Disney to keep the prequels in the lore. Really just GL's last laugh at the fans. "Hate my prequels?! Now they'll NEVER go away!"... GL is the Sidious of the Star Wars Franchise... lol