What you said is true, all altcoins are tied to bitcoin. Usually if bitcoin is up, the altcoins are up and vice versa. However, if bitcoin fails, it can cast a negative shadow across cryptos in general, as bitcoin is the most popular. I think it's better for now that bitcoin survive but it would be fine if another crypto takes over bitcoin's #1 place. This is just my opinion.
Altcoins are tied to bitcoin only for one reason - almost none of them have trading pairs with dollar, and those who entered alcoins for dollars only - they see the up-market and dont sell. And what happens when they realize this is the end of up-market? Yep, they sell, maybe cash out - selling bitcoin too, making it deeper. Thats the simple magic of bitcoin-altcoins dependence