There are quite a few things that 5e could use improvement upon from a (long-time) gamer perspective, and from the point of view of game theory, mechanics, etc. Doesn't mean these things are completely broken or whatnot, just that they exist in the game for reasons other than pure game design.
Note: None of these issues seem to have affected its popularity. D&D is more popular than ever.
In the old days, with a much smaller market of long-time gamers, there would be good reason to re-vamp the game now and then. The audience is waaay beyond that now, though; just as board games made a resurgence a few years ago to a higher degree of popularity than before, D&D has done the same. And done so following the board game model.
There won't be a Catan 2e. Monopoly hasn't changed, nor has Risk...this is it. D&D (why even bother using the "5e" tag?) as it is now is how it will be. Some style variants? Sure (LotR Risk, for example). Slight changes (through errata like the slightly different versions of Fury of Dracula)? Sure.
But sweeping changes like 3e to 4e? 2e to 3e? Even 3e to 3.5? Nah. No money in it. Not now, and not for the conceivable future.
This is the downside to following a game that is designed primarily to be popular, as opposed to a game designed to the most creative ability of talented designers, unfortunately: the different priorities mean that certain things, game design elements, will not go through official change.
Hopefully, UA will move from "playtest ground" to "Showing off my game-design chops" to address this, in the grand scheme of the market, fairly niche desire.