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.
i seriously doubt that there would be any difference between Libra and PayPal. in my view they are exactly the same thing and people (the end users) don't care or even notice what the underlying technology is, whether it is a simple database (aka PayPal) or a blockchain (aka Libra) in the end they will be having an account linked to their identity on a website controlled by a company that acts as their "online bank".
and i don't think they are going to put any kind of links between their coin and bitcoin, their website, their paper,... don't even mention bitcoin. not to mention that it will be centralized.