The more likely reality is that we have a lot of v4-only hw in place with lifespan of 20+ years. Those devices won't go away.
Heck, I work on embedded, and having a dual-stack system is just a PITA to deal with. If v6 would have been fully retro-compatible this wouldn't have been something to think about, but you can't drop v4 and there's no future in sight where v6 will be the only choice (we'll have dual-stack for a looooong time), so we just push the problem up the chain.
There are plenty of systems being developed _now_ which are still v4 only as a result.
Heck, I work on embedded, and having a dual-stack system is just a PITA to deal with. If v6 would have been fully retro-compatible this wouldn't have been something to think about, but you can't drop v4 and there's no future in sight where v6 will be the only choice (we'll have dual-stack for a looooong time), so we just push the problem up the chain.
There are plenty of systems being developed _now_ which are still v4 only as a result.