... which means that if of two parties only one understand UTF-8, they have to speak ASCII. ASCII being a subset of UTF-8 does not provide and benefit in that regard, just as IPv4 addresses being a subset of IPv6 addresses wouldn't.
Imagine two endpoints talking a backward compatible pair of standards: A only speaks ASCII, B only speaks UTF-8. Would you think they will be able to communicate at least something?
Now imagine two endpoints talking a not backward compatible pair: A only speaks IPv4, and B only speaks IPv6. Will they be able to communicate, at least something?
See the difference?
As long as they only speak ASCII, yes.
> Now imagine two endpoints talking a not backward compatible pair: A only speaks IPv4, and B only speaks IPv6. Will they be able to communicate, at least something?
That's why you deploy IPv6 dual-stacked with IPv4. That way, if two parties only speak IPv4, it's completely backwards compatible.
> See the difference?
Sure do I see the difference. What I don't see is how to apply any of that to the v4 to v6 transition in a way that would have any advantage over how it is being done now.