It is possible to track them. Most of it is public and pseudonymous. It's used by criminals not because it's secret, but because it's convenient. For instance, you can get millions of dollars from someone without having to meet up in person to exchange heavy bags of currency or valuable items. Before crypto existed, criminals figured out ways to receive money when it became valuable enough. Now that there is crypto, criminals still do that but have branched out. If crypto dies, criminals will still commit crimes and will still find ways to get their anonymous money.