Both backend and frontend revolve around the API, and Bitwarden owns that not Vaultwarden.
If Bitwarden decides to modify the API so that only their closed-source backend has the full spec, and only gives that spec to their own apps, everything else (Vaultwarden and 3rd-party apps) will fall behind or become unusable.
To counter that, Vaultwarden and the 3rd-party apps need to declare their own API and prepare to fly solo completely independently from Bitwarden.
Such a move would force Bitwarden to show their true colors. If they really mean to stay true to the open version they will keep cooperating and keeping their API in sync with the Vaultwarden API.