I'm not sure you understand what you're proposing. If you end IPv4 support on your product, all you're doing is banning the users on ISPs that don't have IPv6 support.
The people feeling the pain would not be in any position to fix the problem, and their experience will be that your site is down which leads to support burden and reputation risk for your product. If your support tells me to switch ISPs I'm going to roll my eyes and find another product that works.
I interpreted it to be about vendor contracts. Suppose you're setting up a new thing and you have a choice of vendors. They're all about the same but one of them supports IPv6. You're more likely to pick that one.
No, but imagine if Google, Meta and Netflix all publicly agreed to stop supporting IPv4 in X years.
_Everybody_ would rush and make sure to switch everything to IPv6.