It is very hard that Bitcoin crashes back to 5000$, but assuming that this could happen someday, it will affect hardly the whole crypto market for sure. We knew, depending on our experience in trading BTC and altcoins, that BTC price and chart are affecting most altcoins markets in the previous years. So I guess most altcoins prices will decrease even more if BTC price dropped down to $5k.
I believe it is fairly possible. Considering how it dropped from $20k to $13k in a matter of 5 days in 2017, it happened. It may happen again, it may not. However, you're on point about how Bitcoin affects the altcoin market, as a person who had various altcoins a few years back when it (Bitcoin) crashed down hard to about 10k I ended up losing a fair amount of my bank. Now, the only thing I trust in is Ethereum and thats about it.