Seems you're misunderstanding what stablecoins were developed for. It was developed with the intention to save your investment from crashing down due to a bear market and not to give you profits. Irrespective of the initial motives behind creation of stablecoins, it can still be used as an investment vehicles if only you're opportune to catch dip of the stablecoins and invest when everyone is panic selling their stablecoins.

You're better off patronizing any other stablecoin than Libra (that's if the project successfully gets the approval of the US government and launches). It'll just be a centralized piece of trash and your security won't be respected just as it's with your fiat currency.