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.