There's no need to be problematic about when Bitcoin becomes a legal tender if you're worried about the altcoins. We did already see this happen to El Salvador and to the Central African Republic and guess what happened? Got nothing for the altcoins. But as a thought of what usually happens to the alts when Bitcoin pumps, then we've already seen that it's making a huge round on what actually happens for many altcoins but not for the most, they also pump.
As long as bitcoin keeps doing halving mechanism and the chance for altcoin to increase will always there.
Well, the direct impact and effect goes to Bitcoin but as a common reaction of many altcoins, they are also benefiting from Bitcoin's halving as the market goes in the bull run so they do as well.