1. Peer-to-peer networking won't usually work correctly. And quite a bit of software uses P2P networking these days---BitTorrent, Zoom/Teams (via WebRTC), Tailscale, PlayStation/Xbox multiplayer, etc. Most of these services have automatic fallbacks when P2P networking doesn't work, but these fallbacks are usually slower and less reliable.
2. Most websites assume that 1 IPv4 address==1 household, so you'll often run into rate limits. Or even worse, you might be blocked entirely if your CGNAT neighbours are spammers or otherwise breaking website rules.
1) my stateful firewall is going to break most of that anyway
2) if cg nat is as popular as people claim then they won’t be doing that as it’s not an edge case
While true, neither of those are relevant in context (and I even explicitly acknowledged your first bullet in my comment above). It was suggested that a website operator deploying IPv6 would somehow improve the end user experience by virtue of avoiding CGNAT and I was questioning that. I do of course appreciate that going via CGNAT to a clueless operator that eagerly adds IPv4 bans can be problematic but that's more a question of why you as a consumer might want IPv6 connectivity not why a service provider would want to deploy it.