It seems really naive to think that if FB were to get a foothold in the currency markets with a permissioned product that they would later on go against their own interests and transition to a permissionless alternative (not to mention the massive legal and technical hurdles that this transition would require)
...but I don't know why I'm arguing against Libra, it's such an all-around terrible product, I should just keep my mouth shut. Facebook is tarnishing the idea of a "corporatecoin" for the next few years and doing all of us a favor in the process.
Of course, they were very happy to make all kinds of vague, rosy promises right out of the gate... it doesn't cost them anything to do so.
But if you are capable of doing an initial launch on an permissioned ledger and achieve the initial network effects anyway, what's the point of providing such guarantees later on?
Call it like it is: Company scrip.