Who's arguing for that? That would be completely non-viable even today, and even with NAT64 it would be annoying.
> Dual-stack fails miserably when the newer stack is incompatible with the older one.
Does it? All my clients and servers are dual stack.
> With a stack that extends the old stack, you always have something to fallback to.
Yes, v4/v6 dual stack is indeed great!
> To replace something, you embrace it and extend it so the old version can be effectively phrased out.
Some changes unfortunately really are breaking. Sometimes you can do a flag day, sometimes you drag out the migration over years or decades, sometimes you get something in between.
We'll probably be done in a few more decades, hopefully sooner. I don't see how else it could have realistically worked, other than maybe through top-down decree, which might just have wasted more resources than the transition we ended up with.
I don't see IPv4 going away within the next fifty years. I'd not be surprised for it to last for the next hundred+ years. I expect to see more and more residential ISPs provide their customers with globally-routable IPv6 service and put their customers behind IPv4 CGNs (or whatever the reasonable "Give the customer's edge router a not-globally-routable IPv4 address, but serve its traffic with IPv6 infrastructure" mechanism to use is). That IPv4 space will get freed up to use in IPv4-only publicly-facing services in datacenters.
There's IPv4-only software out there, and I expect that it will outlive everyone who's reading this site today. That's fine. What matters is getting proper IPv6 service to every Internet-connected site on (and off) the planet.