The mainstream could continue paying with Paypal and credit cards since it's all the same anyway. This way, they won't even need to pre-convert any money to Libra.
If it works similarly to how for example the stablecoins we have here work, it's actually quite an improvement over the services offered by financial institutions such as PayPal.
Don't forget that PayPal is a safe haven for scammers with how their bias always leans towards the buyer. I'm certain Libra in working condition would greatly exceed the workings of PayPal & Co. PayPal is shit all the way through. It costs me like $4 to send a sub $50 sum of money to someone in another country. Libra will demolish this house of cards. I welcome it.
Also, Libra is an amazing tool if it's paired against Bitcoin because that means a new stream of liquidity finding its way to Bitcoin.