Since Libra isn't distributed, in the sense that anyone can start mining it, one of the key premises for its trust was that governance and operation will be handled by a group of powerful and trustworthy companies.

The more companies leave the initiative the more centralized it becomes and the less its trust model is valid. If only Facebook remains it isn't any different than a simple virtual currency (like the discontinued Facebook Credits).

And to preempt the argument that visa and mastercard aren't trustworthy - there is an inherent trust that most people give to big companies because their processes and checks and balances developed over years to be robust - usually from having to handle lawsuits and regulations which small companies rarely do.