It was easy to notice that Bitcoin fell in tandem with the stock market when Covid started happening, so we should not be surprised in future if a worldwide recession caused by say - a heavily mutated version of that virus which can invade current vaccines, was able to exasperate the current stretched economic problems. I'm not sure that the sheer number of cryptocurrencies will matter all that much, because you'll tend to find the 90%+ of the money will be concentrated in say the top 100 biggest ones. All the others were likely to bust anyway, even if they never boomed in the interim. Environmental pressures on certain energy intensive cryptos and governments competing to launch their own cryptocurrencies could also make a dent in future, but it'll probably be short lived.