Perhaps a silly question as I do not have a lot of experience with software like this, but:

What prevents Lastpass, bitwarden or any other third-party to update their software (and/or compromise the download server) to synchronize all information un-encrypted in a new version which is auto-updated by the user?

I currently use KeePassX, and synchronize this file with a secure server myself since I feel uncomfortable with having software that handles the encryption also controlling the synchronizing service.