Actually, bluesky has supported custom domains since the very beginning (the Namecheap deal is the part of this story that is actually new/noteworthy) and it is very widely-used. I am unsure what % of active users are using their own domain, but it is not insignificant.

The domain feature hasn't blocked dev on more important things. In fact this deal is (according to bluesky) their first attempt at something to generate some revenue, with the goal being to never have to use advertising, something that the users would in fact be grateful for and consider to be important.