I generally feel positive about something like Libra and I'm not in favour of decentralised currency. Decentralised currency is hard to tax, threatens the sovereignty of nation states, the ability to enact sanctions, enables crime and I'm not really on board with that.
Libra (or something like it) looks like a way to built a cheap, frictionless payment that in any practical sense mostly seems to illemininate costs of transaction if it scales up well enough, and I think that's where most utility comes from in a currency. I don't see the value in some slow network that changes its value twenty times per month for the average person just so that you can stick it to the man or whatever.
As to 'controlling the world economy'. I've never really understood how this is supposed to be the result of Libra. At the end of the day it's basically a Paypal without the middleman. The Libra owners will at some point earn some money on interest but otherwise I don't really see how this confers any monstrous powers to them. As far as that goes I'm more concerned about the Facebook platform itself.