Just want to point out it's not quite that easy: If you convert 1.24 (or whatever) bitcoins into zcash/monero/etc and then 1.24 right back into bitcoins, everyone will know those transactions were likely connected.
I should think that it would be obvious that you don't put in 1.24 BTC and then take out 1.24 BTC later. Someone trying to hide the source of their funds via a mixer or Monero or whatever would put in 1.24 BTC and take out 1 BTC + 0.2 BTC + 2*(0.02 BTC) (or some other set of common denominations) in separate transactions to distinct BTC wallets, taking steps to ensure that these transactions can't be correlated with each other by IP address, time, etc. They want to hide the withdrawals in a sea of indistinguishable, uniform transactions; moving a unique, traceable quantity all at once would be a clear red flag. The principle is hardly obscure; it's exactly the same as the popular image of criminals conducting business with suitcases full of unmarked $20 bills. They use $20 bills because they're commonplace, not because they're more convenient than $50 or $100 bills.
