I am guessing the reasoning goes like this: if we only support IPv4, then it's on all these ISPs with IPv6-to-IPv4 CGNAT to make sure their stuff works properly

I am guessing the reasoning goes like this: if we only support IPv4, then it's on all these ISPs with IPv6-to-IPv4 CGNAT to make sure their stuff works properly (and chances are they will notice if it doesn't). But if we support IPv6, it will be on us to make sure people behind those ISPs can still reach us (because hardly anyone goes via this IPv6 route and if it doesn't work it's likely nobody at the ISP will notice).