Clients are OSS, I wonder why nobody did a Vaultwarden-style fork of them yet that would watch over upstream changes.

Until Bitwarden screws up it's going to be difficult for any fork to get much attention. If they do, that will the moment to launch a fork.

It's Bitwarden's game to lose. Forking is easy enough that there's no great need to pre-emptively fork.

Vaultwarden is a very lean implementation of Bitwarden but if you want to look into an alternative to the Bitwarden ecosystem, I recommend - AliasVault https://github.com/aliasvault/aliasvault - check it out!