I had no idea we were supposed to preface our posts with "This is just my opinion but, "
This is JMHO, but...
Sounds like a band-aid fix more than anything. First, accelerating the levels to reach the higher ones faster just makes the leveling process pointless.
Leveling isn't even, either. The first few levels go very fast, then they slow down through about 11th, then speed up again. The exp chart focuses campaigns on the 'sweet spot' mid-levels where 5e works best. Speeding to the higher levels won't make the higher levels any better. Though it will, in theory, address the complaint that any defining higher level abilities don't come up soon enough, it'll also mean you don't get to use them as long, since you spend less time playing at those levels (less time on the campaign, generally).
Second -- and more importantly here -- it does nothing to address the fact that running high-level adventures (T3 and T4) is much more difficult than the early stuff, due to the way magic trivializes all the types of challenges that we associate with good adventures.
Nod, that's always been the case, even when high levels were least problematic, characters could simply do more at those levels and running the game was more challenging. Of course, you get some experience DMing as they go up in levels, and the players & the DM, in a conventional up-from-1st campaign, learn the PCs' abilities very well on the way to high level, so that should help.
It's kind of like the funny comment about why Gandalf didn't just have the giant eagles fly to Mordor in the first place. People like try and come up with all kinds of reasons why (too dangerous, they aren't bound to his will, etc) but it really just comes down to this: there would have been no story.
It's a style thing. D&D lets you go with that kind of 'what if,' exploration of fiction, you can take the stereotypical fantasy formula and turn it on it's ear, or play straight with it, depending on your whim.
It all really does still just come down to problem with the spell lists. They were broken back in 1-2e at later levels because Wizards got stupidly powerful, but at least then the game didn't assume (or even encourage) high level play as part of the expected experience.
It presented rules for high level play, including exp tables that went to quite high levels & a wizard spell/day chart that went into the 20s, arguably 'high-level' monsters, tons of magic items, etc.
With later editions trying to encourage the full 1-20 experience by virtue of giving non-spellcasters cool new powers later on to keep up,
3.5 gave fighters some feats that required +8 BAB or a minimum fighter level, but the last 1/4 of the 1-20 experience were just picking up feats they'd passed on in the past. Of course, 3.5 went Epic, too. Besides, earlier editions gave the non-casters cool items to keep up at high level, in theory, anyway, it kinda depended on how you rolled on those treasure tables, but they were weighted that way.
yet often showing zero guidance on how to actually run those later levels, they've painted themselves back into a corner.
In 5e there's only so much guidance you can give - and precious little thanks you get for giving it (6-8 encounter days? just say'n) - there are encounter guidelines, but they're not dependable at any level. It's just a matter of DM talent/experience making the game work at high level. I doubt it's a coincidence that APs run their course in fewer than 20 levels.
