As you know Facebook announced their own cryptocurrency LIBRA but various legislation needs to complete before launched, so after launch can buy but this will be stable coin and will not affect BTC price.
Directly it won't, but indirectly it will, once people get educated with what is libra and eventually use it, they will be introduce in the crypto market and they will surely know about BTC which is a good investment, not only that, this success of Libra will also become a great help for altcoins to recover as investors would certainly touch them knowing they are so much undervalued.
Your arguments sound weird. How is Libra going to help the shitcoins? And I am quite sure that by the time Libra comes to existence (mid-2020), many of the shitcoins will cease to exist. Libra can be a competitor to the altcoins (especially for stablecoins such as USDT, USDC and PAX). I can't imagine a scenario in which the introduction of Libra is going to help the alts in regaining their lost value. It may help Bitcoin, because many of the Libra users may shift to BTC in the future. But I don't think that any of them will shift to altcoins.
In case everything goes as planned for Libra, then the coin will be introduced sometime in 2020. The point to remember here is that Libra is supposed to be a centralized stablecoin, unlike Bitcoin which is a decentralized dynamic coin. Many of the existing FB users will start using Libra (and the vast majority will be using crypto for the first time in their life), and after a while many of them will start exploring BTC. Introduction of Libra can result in more users to BTC in the long run.