[yawaramin]

> However, non-current packages may break if they're missing upper bounds....

Out of curiosity, how feasible is it for opam packages to use lockfiles like npm is nowadays? Then packages which depend on some other package will continue to build against the exact same version they always did, until and unless they are explicitly upgraded to the latest version of that dependency.

> I'm not sure why it would build the same dep twice - I haven't seen it do that.

Maybe https://discuss.ocaml.org/t/why-does-opam-recompile-so-much/... provides a clue to that.