I can't agree more，we have to stay calm when the market turns red. Only in this way we can make an wisest decision, or we will probably sell our coins with the public panic. Of course, as you've said，we could even invest in BTC or ETH when the market falls if we have spare money. But if we dont have, what we need to do is just holding our coins tightly.
Beauty of crypto is to make you a profitable person in every situation and I really like this thing. For bitcoin case specifically, if market is down, dont just get panicked. You have got some options still. Either you for making a good buying for bitcoin and start holding it or short term trading or you just get things on hold. Whatever you have, hold it and dont be panic. In both situations results are in your favor.