5e was a huge opportunity to do some modernizing to the traditional ruleset of dungeons and dragons.
No, no it wasn't. 5e is fundamentally a revival product, bringing back a fad from 30-20 years ago. It re-wound a lot of 'modernization,' fed hamburger into a meat-grinder in reverse and popped sacred cows out the other end with preternatural success.
It didn't just re-print the traditional ruleset with new art, though, so yes, there's some cool stuff preserved from d20 and even built upon, a little. But if you judge it by how 'modern' it is, you're really not being fair to it, at all.
However, my biggest complaint by far about 5e is how they made almost no updates to the traditional attribute system d&D has always used, which frankly has a lot of problems.
I can't imagine how that'd remotely have been on the table. 5e was absolutely committed to re-capturing the feel (and lapsed fans) of the 20th century editions of the game. Attributes are one of the few things that weren't terribly desecrated by the sacrilege of 3e & 4e modernization. They probably regarded them as some sort of untouchable 'third rail.' You could no more get away with changing the traditional attributes than you could switch from d20 to percentile. Be thankful they didn't go back to arbitrarily different bonuses for each stat, and kept the +1 mod per 2-above-10 formula (BTW, first introduced in the 4th ed of Gamma World, c1992, iirc).
Pillars of Eternity is a great example of how the 6-score system of D&D could have been easily updated into something more coherent, sensible, modern, and balanced.
For every rule between the covers of a D&D book, there is some other game some where that does that particular sort of rule way better - but D&D can't afford to go emulating that other game,
because it's not D&D.
There's no such thing as a dump stat. Some stats may be more useful for some builds than other stats, but it comes much closer to being class-independent than D&D.
There are a lot of negatives to class system, but one of the positives is that they do simplify the decisions of character generation. Obvious 'prime' and 'dump' stats are part of that. 5e does go further than prior editions in trying to make each stat relevant to all characters, though, with all six being used for saving throws (however rarely half of them get used), and with proficiency/level bonuses being small enough, and DCs 'bounded' enough, that a decent stat mod is always helpful. So they really did make an effort, there.
I definitely think that they didn't streamline the magic system as much as they could have. Sure, it's way more forgiving in terms of spell preparation, but not more than 3E was more forgiving than 2E.
They could have ditched spell preparation for all classes, but they didn't, and that makes me sad.
In favor of what, exactly? Spontaneous casting of any spell on your class list?
There are thousands of RPGs out there, and they pretty much all use different attributes/stats/ability scores. You know why?
Copyright law? ;P
You know what they got wrong? Not pleasing everyone. Seriously, what were they thinking?
That'd've been funnier if pleasing everyone ("who ever loved D&D," that is - I guess if you're not pleased by 5e you never really loved D&D, you were just leading it on?) hadn't been a goal from the first announcement of 'Next.'