Specifically, if a plain router stops working, BGP will route around it, and all you did was drop packets. If NAT stops working, you don’t just drop packets, you drop whole connections. Applications don’t tend to tolerate dropped connections as well as they tolerate dropped packets.
That is the biggest problem with IPv6. Who is gonna be the first ISP to shut off their IPv4 stack? There is always gonna be some random website that is IPv4.
You might just direct all the v4 traffic via a tunnel to another ISP which specializes in legacy services like IPv4, running SMTP/news servers, etc. Now you've saved all the cost of maintaining all the IPv4 peerings and config.
Well I have been hearing about the end of IPv4, and IP exhaustion for about 20 years now, and I fully expect people to still being moaning about it 20+ years from now while the majority of the interment still communicates over ipv4
v6 also has better measured performance on webpage load times. Perhaps "pages load slightly slower than they could do" isn't a show-stopping problem, but faster would still be better, right?
How?
IPv6 isn't a substitution for CGNAT, it's an addition to it. You either have to keep CGNAT or replace it with dedicated IPv4 for each customer. Dedicated IPv4 is most likely too costly given the limited availability. SpaceX is also trying to cut cost aggressively.