I didn't say it's gonna help shitcoins, altcoins that aren't performing yet are not automatic shitcoins.
I know that Libra is a stable coin but it's popularity will help people to be aware of the crypto market because it's still being traded here in the space in different exchanges, probably the big ones.
People when start using Libra will not close their eyes and will focus on using Libra alone, they will become curious with the existence of BTC and altcoins, and eventually will explore it. As you said, it could affect BTC, so if BTC will rise and hopefully make a new ATH if it will not happen this year, it's expected that altcoins will follow, based on the price trends in the past.
Those who start to use Libra will eventually come to know about Bitcoin, and many of them would start using it in the future. But I don't think that the same is going to happen with altcoins. Bitcoin dominance has already hit 69%, and altcoins are drowning without any sign of a fight-back. By the time Libra is listed, many of the altcoins would be either dead or dormant.
There are altcoins out there which are centralized, similar to Libra. One example of such a coin is Ripple (XRP). There is a chance that the emergence of Libra may give XRP another round of pump-and-dump. But there is zero chance of XRP holding on to the gains, simply because it is a scam coin. IMO, some of the Libra users may start using other cryptos, including alts. But in the end, most of them will realize that holding alts is not worth the risk.