I think that when Bitcoin becomes legal tender, alt coins will experience both growth and volatility. Some people will definitely jump into Bitcoin, but others will stay invested in alt coins that they believe in. It will ultimately depend on the specific alt coin and its use case.
Altcoin may also be considered as legal tender as well. it may not applicable for US but it will be for another country. I meant if we must also realize how bitcoin is always trying to bet even better and better. Usability from altcoins are not greatly impacting the performance from the altcoin itself. As long as bitcoin will able consistently to increase steadily. Altcoin will be facing the same patter like bitcoin. I hope that more countries will consider bitcoin as legal tender. this greatly impacting altcoins.