This is a very common misconception. The issue is not IPv4 or CGNAT, it's stateful middleboxes... of which IPv6 has plenty.

The largest IPv6 deployments in the world are mobile carriers, which are full of stateful firewalls, DPI, and mid-path translation. The difference is that when connections drop it gets blamed on the wireless rather than the network infrastructure.

Also, fun fact: net.ipv4.tcp_keepalive_* applies to IPv6 too. The "ipv4" is just a naming artifact.