I missed a couple of things in the OP when I wrote my previous post in this thread...
A D&D that goes to level 10, would actually be best. As a matter of fact I'd rather they focus on levels one to ten in the PHB (with a ton of options) they publish and release levels 10+ later down the road.
There's a problem with this, in that high-level play requires quite a lot of careful design work. And if that work isn't done when the system as a whole is constructed, it's very likely that it can't be done well later on. In fact, it's quite evident in 3e that a huge amount of work went into playtesting the low to mid levels, and much less at higher levels. The rules give an impression of a mathematical rigour that they don't, in fact, possess. (In 3e, this is also true of the item creation rules.)
In order to do high-level play 'right', it is probably necessary that they do the bulk of the design work for it right at the outset of the edition. In fact, I'm more than a little concerned that the existing playtests have explicitly and deliberately focused on low-level play.
The problem with level ten plus that I am seeing is that they could in theory be done at any level. Rule a keep? Could be done at level one depending on the campaign. Travel to the planes? The same.
...
What about your high level games says "this is high level D&D done right" ?
Well, I'm not sure if what we do counts as "doing it right", but...
As the game has moved from low- to mid-level, and again from mid- to high-level, I've tried to consciously change the scope of the game. Low-level is all about dealing with local troubles and small-scale issues - the PCs run into bandits, and thugs, and the occasional band of orcs. Mid-level is very much quest-based, and the threats another step up.
But the high-level adventures are the truly world-shaking stuff - the PCs are the biggest and the best heroes around. The trivial concerns of lesser characters just aren't an issue for them - they can take on entire armies of orcs, they can stand toe-to-toe with demons, dragons, and even demigods if the situation warrants. And the outcome of their adventures may permanently alter the game world, for good or ill.
So, while it's true that a party
could rule a keep or go planar hopping from first level, IMC they simply
don't.
(For another shorthand about the difference between 'tiers' - low-level is "save the village", mid-level is "save the nation", high-level is "save the world". Or destroy it, of course!)