it is very difficult for any coin to replace Bitcoin because almost all coin benchmarks are in Bitcoin.
...and the US Dollar; also, this wouldn't matter. Why? Because like I said in a previous reply[1], Libra isn't focusing on being a stablecoin to be traded on exchanges. Libra is focused on global adoption, straight off. And just based off that fact alone, it's pretty easy to say that Libra can easily overtake Bitcoin in terms of transaction volume, marketcap, and adoption in general.
[1]
https://bitcointalk.org/index.php?topic=5156459.msg51534561#msg51534561