I don't think you could design it in such a way that it's easier to migrate to.

It's a fundamental change at the IP protocol layer, there wasn't scope in that layer for backwards compatibility.

There's loads of transition mechanisms that were introduced, but it's not as tractable of a problem. Assumptions around IPv4 have been baked in all over the place.

Also there was more _reason_ to move to HTTPS. Virtually everything had already supported HTTPS for closing in on two decades at the point the mass rollout with LE happened. You could switch to HTTPS-only for your site without breaking it for your users because they already needed it for online banking etc.

You can't just switch to exclusive IPv6 because half your users don't even have an IPv6 connection. And virtually everyone with IPv6 has an IPv4 connection anyway so the benefit in dual stacking is nil. Unlike supporting HTTPS where at least those that supported it got the benefit.