If you deploy IPv6, you might not deploy CGNAT, but unless you are dual stacking, something functionally equivalent is required.

As to scaling costs, IPv6 offload isn’t a 1:1 replacement for IPv4 or CGNAT capacity. You need enough capacity to carry the IPv6 traffic over IPv4 in case your IPv6 transport, transit or peers fail. Again, not optional.

Thus in a properly designed network these are not optional costs.

The failure mode for your v6 failing is the same as it is for v4 failing in a v4-only ISP: stuff breaks. If you want to fix that you need a redundant failover setup, which is the same thing you need in v4 except cheaper because it doesn't need to do CGNAT.

Surely you need enough IPv6 capacity incase your IPv4 transit fails as well. If JIO could have built enough IPv4 capacity to run their network over IPv4, they would have, but they can't, so they've been pushing IPv6 hard.

I’m not familiar with JIO’s setup, but that’s an interesting question. If their IPv6 was down, would their network still be able to deliver? Would they run out of CGNAT capacity or IPv4 translations?

IPv6 is no longer optional for them. I suspect it's not optional for T-Mobile USA either. Or Facebook/Google/Netflix.

Or have I been living with Canadian ISPs too long and that sort of thing doesn't happen in the rest of the world?