The problem is that (1) can not happen everywhere at once. So your “128bit IPv4” stack still needs a way to talk to “32 bit IPv4” stack, so you need to maintain somewhere a flag which one do you use.

So the problem is exactly the same as with the transition - you need dual stack everywhere, and a means to select which one to use.

I think the simplest analogy of this that makes the complexities more obvious to understand:

Imagine you have a network based on copper. (Twisted pair). Please upgrade the entire network to the optical fiber while maintaining the transparent connectivity between all the hosts at all times.

Edit to add:

bad analogy. Copper or fiber are sufficiently non-leaky to make the task relatively simple. Maybe a better one could be to migrate the hosts from IP to IPX. (Which is kinda similar to IPv6 sufficiently that it renders the analogy moot :)

This is exactly how Happy Eyeballs works: request AAAA and A, and use AAAA if it returns.

Trying to force things one way or another is a recipe for a bad user experience.