What happens if the moon is made of cheese?

Outside of obscure hypotheticals you make do or switch providers.

It's not that obscure. It's common to be given a /64 to a home, or even to small business premises. Some even get a /128.

So it's reasonable to wonder how people deal with that problem when needing multiple subnets. Many locations don't have a realistic choice of affordable providers, or none of them provide IPv6 with a shorter prefix anyway.

Depends on what you want to do. Maybe use one network or assign static addresses. Here's a post on /120 networks: https://blog.ipspace.net/2011/05/ipv6-neighbor-discovery-exh...

My ISP doesn't even give me a /32 v4 or a /128 v6. CGNAT v4 only.

I'd love to have to make do with a /64.

My ISP did exactly that. I bugged their customer support several times over a month and they finally caved and gave me /60 now.

My ISP gives me /64. It is unusable with consumer routers.