My prediction [0]: It will take roughly 100 years for IPv6 to be ubiquitous enough to shut off IPv4. That's not intended as hyperbole, if anything it's an understatement.

Because, it's not going away: You can talk all you want about how IPv6 should have been a more straightforward expansion of the address size, but this is all in the rear-view mirror at this point. IPv6 is going to be with us forever, you may as well get used to it. It's already everywhere in 5G deployments, ISP's like Comcast use it for 100% of their out-of-band management, China is making huge progress moving to it as part of their 5-year plan, India is progressing nicely in their transition, the list goes on. We're already way too far along in the transition to abandon it in favor of something else.

But it's not going to happen any quicker than we've seen, either: There's no urgency (no "must-have" use case) except for what organizations are imposing on themselves. Yeah, IPv4 addresses are more expensive, but you don't really need many of them as a business (you can get by with a small handful of public ones, and just using L7 load balancers and SNI for everything) nor as an ISP (CGNAT can get you a long way.)

So we have a situation where things are migrating very slowly, mainly only in places where it makes sense (mobile deployments, home ISP's where the users don't actually administer the network), and generally mostly for new deployments. This is a recipe for IPv4 to be around for a very, very long time. We're used to technology moving at breakneck pace, but that's only the case for the higher-level stuff. The core infrastructure like the internet protocol is likely the textbook example of slow-and-steady, and a case where it's actually not crazy to think of centuries-long timeframes for things.

[0] Barring any unforeseen black-swan events like a world war destroying all technology and having to rebuild from scratch or something. Or a competent international agreement to aggressively migrate to it (I don't know which is more likely.)