And you already don't have to use all 128 bits in v6. You're forced to use about 48 of them, but the rest is your choice. You can just... not use some of them. It's fine.
You're kinda forced to use 128 bits in v6 if you're an end user, cause that's the lowest common denominator. SLAAC is default, DHCP6 is widely unsupported. Most ISPs also only give out a /64 per spec minimum.