Libra will be centralized and regulated by bank. Even though it has the concept of a cryptocurrency, it might not behave like it. It might get a lot of users, because facebook is a social networking king.
But users of Btc, and other cryptocurrencies wont just adhere to Libra if they see it has a normal currency. And being Libra the 1st alleged cryptocoin that will be connected with USD, it might increase the value to USD in relation to other fiat coins.
In the end, we shall  wait to see what Libra can affect and how it will affect.