Based on observations/listening experiences to date, I'm finding good Class D seems to sound more like good tube amps I have heard than most other SS amp technology. Plus they have the ability to drive many difficult load speakers out there today that tube amps are more challenged to do, without great cost, expense, size and heat.
Based on listening alone, I find my Class D amps, Bel Canto ref1000m monoblocks, hard to fault in any real way. But on paper, the very highest frequencies that might be heard are the area where Class D historically may not be up to snuff with the very best amps otherwise. That technical bottleneck as I understand it is due mostly to switching frequency limit and effects on associated low pass filtering needed. The latest Class D amp modules I read about seem to indicate that switching frequency continues to increase and improve as the needed technology improves. That would seem to push what is possible even higher.
Practically, I think Class D has arrived not just as being viable but the most practical approach to high performance amp design. Technically, the frontier is still moving with Class D technology it would seem, much as it is still with computers and related digital technologies.
The sky is probably the limit down the road, to the extent that it matters practically. Practically, what it means is though performance will continue to go up, becoming even more practical for even the most demanding applications, even more importantly, cost/prices will continue to go down as top performing Class D technology becomes more mainstream for high end audio applications.
Outside of high end audio, where Class D appears to already have a firm foothold, Class D seems to have already clearly arrived and is starting to take over.