I think "one D&D" is a huge mistake.
Sure they can maintain 5e with some changes, but they should also be working on 6e, and incompatible, reinvented version with new mechanics. The thought that you can't make a better game is ridiculous.
What makes something a "better" game, though? That is always going to be a subjective question. Yeah, they could throw everything out to work on making a better game for
you, but that might make it a worse game for
me. In fact, that is pretty much exactly what they did to their base when they went from 3rd to 4th edition.
Honestly, put yourself in their shoes. You've finally got a version of the game that has been met with widespread acclaim and extraordinary sales. Are you really going to throw that out in hopes of making something totally different and maybe "better"? For what? To please a few disgruntled fans? That would be crazy. You will
always have a few disgruntled fans.
So they are going to stick with what is working and just tinker a bit, leaving the door open to future tinkering. Some of the changes just reflect changes to the culture at large ("race" becoming species; alignment becoming more optional). Others reflect adjustments to problems that have only become apparent over time (rebalancing a few feats to make them more evenly competitive). Others seem like corrections to earlier oversights (
spiritual hammer becoming a concentration spell). Others reflect design choices that have become more comfortable over time (feats going from optional to more integral). Others are intended to improve consistency and simplify the game (all subclasses begin at level 3). And so on.
Taken together, I agree that none of these are revolutionary proposals, but they do represent an attempt to evolve the game in a positive direction. And that's all I want. Like many, I am satisfied with the 5e chassis. It feels like the D&D I played in 1979, but with more consistency and clarity. Keeping 5e but not freezing it, so that it can slowly change with the times rather than in a series of jarring edition jumps, makes sense.
WotC has chosen the metaphor of evolution for what they are trying to do with OneD&D. I think that is a great metaphor, because in evolution, there isn't a sudden, complete transition from one species into another, there is just incremental change over time with no clear dividing line. Decades hence, D&D might look quite different from what it is today, but there won't have been a specific moment where all your old books became obsolete. The scope of the changes will just be apparent when you, say, hold up your 2014 PHB next to your 2064 PHB.
To make that happen, they will have to disappoint folks who look forward to getting a whole new edition at once, like in the old days. They are okay with that. So am I.