If Libra becomes a reality, then it might compete with the XRP ledger as we know it. Of course, Libra will be a stable coin whereas XRP will remain as a speculative cryptocurrency. But I believe that Libra will directly compete with XRP in its technical aspects. It will only be a matter of which cryptocurrency is the most scalable one for the mainstream world. The main advantage XRP has on top of Libra, is the first-mover advantage on the market. It already has obtained many partnerships from banks worldwide. If Libra wants to take down XRP's place, then it would need a lot of effort for sure.
Considering that both cryptos are centralized, they will fit the bill nicely towards transforming the world's economy as we know it. But they cannot destroy Bitcoin or other well-known cryptocurrencies because of their decentralized design. Both Libra and XRP's success will not have any effect on Bitcoin or altcoins whatsoever. But be aware, that Libra's popularity within the mainstream world might help increase Blockchain adoption towards astronomical levels. It's only a matter of time before Libra launches by 2020 (hopefully) in order to rival the existence of XRP. Even if it doesn't overcome XRP, both ledgers could exist alongside each other for years to come.
Nonetheless, time will tell us what will be the fate of these centralized cryptocurrencies within the mainstream world. Just my opinion