I dont buy the idea that Paypal added critical legitimacy to the project. SV companies are a pretty homogeneous interest group, and FB is already one of the largest players. For me libra is trustworthy because it's backed by real money from many governments. It's highly unlikely that facebook would commit real fraud with it, since they both have the cash to pull it through and committing fraud would be devastating to their brand (HN may not like FB , but it is obviously one of the better trusted tech companies).
This also opens the way for other more obscure partners to enter the association. That will probably determine its future course
I've got to wonder if Visa, Mastercard, Paypal, Stripe and others got involved with this on the principle "Keep your friends close and your enemies closer"? Facebook is too big to just crush, so you need a smarter approach.
By initially supporting Libra, they got to find out FB's plans, get access to a bunch of market research and see what kind of hurdles they would face if they wanted to launch their own version of the same thing. Then by backing out at the 11th hour, they're able to undermine Libra's credibility before it gets started and make it look like a doomed project.
This also opens the way for other more obscure partners to enter the association. That will probably determine its future course