That exaclty Bitcoin will not defeated by Libra.
Libra is the stable coin for the community of facebook,imo.
They cant go and beat bitcoin as easy just like the hype, they have to be more a stable coin so many users will use their coin on their platform and thats why they created Libra coin just like that. This will still be a big coin but not a big threat, bitcoin knows it.
I suppose both cryptocurrency will not be rivals but rather they would complement each other since both reinforces the notion that cryptocurrencies are an inevitable reality in our near future. I think this will be the beginning of a significant milestone in mass adoption we've never seen before. Imho.
Its the function of altcoins, to complement with bitcoin, the adoption is already happening but it will become big when Libra coin takes the market and also other big companies in the future.