So you migrate to int64 and one day someone will wonder why the hell did we ever think no one would reach 2^64 rows in a database table. Or that 2^128 IP addresses would be enough for everyone.
Yes 128 bits is only 4x times the bit length, but the address space is exponentially bigger.
Some predict if we do run out, it might take ~100 years.
You think we can use 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses in 100 years? Thats ... ahem, ambitious!
And the subnets are in a hierarchy that helps with routing but limits the packing efficiency.
We could feasibly "run out" to the point that we can't keep doing things this way.
Been through it twice in my career…
No process that touches the real world gets into 2^64. No process at all gets into 2^128, at lwast not while we live around a single star.