> What measurable benefit does the user see that hasn’t been a solved problem since Windows XP?

Things being able to connect, and not having to manually port forward (when that's even an option).

Hole punching is super unreliable with CGNAT.

> Will my teams calls suddenly stop saying “poor network connection” on my 1000/1000 rock solid fibre connection?

I don't know how Teams relays data, but for some services yes that could happen if IPv4 can't make a direct connection.

> Will torrents suddenly find more seeds and peers?

Yes. In a typical torrent an annoyingly small fraction of seeds and peers can receive connections. If you're IPv4-only behind CGNAT, you can't connect to them and they can't connect to you. IPv6 opens up a lot more links.

> Will my games… have lower latency?

It depends on how the game is designed. But some games will have lower latency because they can connect people directly instead of with relays.