In fact, one of the main reasons the NixOS Foundation was created was to ensure continuity of the infrastructure by having the financial reserves to deal with sponsoring of the binary cache ending. The foundation currently has ~€230K in the bank, so we are prepared for this.
Moreover, that $32K is a worst case that only happens if we decide to move all of the binary cache out of S3 and we have to pay for it. There are much cheaper scenarios, e.g. we stay on S3 and we garbage-collect the binary cache. (As I described here, keeping all NixOS release closures ever made and deleting everything else shrinks the binary cache to about ~6 of its current size.)