IMO, one of the budget storage companies (Backblaze, Storj, etc) and/or a CDN stepping up to support it out of their marketing budget.

It seems to me that this "hash all the things" setup seems absolutely made for IPFS integration, which AIUI would mean that everyone who chooses to seed those nix stores would incrementally reduce the bandwidth burden upon the "main" nix store. I'm not sure if nix hashes take the $(uname -sm) into consideration, so that could be a hurdle, too

I can very easily imagine that it would need to be opt-in, to keep security peeps from losing their minds, but my suspicion is that a non-zero number of actual nix users would choose to turn it on in support of the community

You propably have to pull all your sources from github and compile them yourself once, since there is no remote store that provides prebuild artifacts? (I dont know what use the S3 bucket/s is for.)

As far as I remember there was enough money to keep them alive on current setting about a year. Worst case they'll switch to an alternative provider which _might_ be a bit slower?

Binary cache for old builds will be lost, they move to some other storage solution, but they can't transfer the binary caches due to AWS's egress costs.