matter
Yes, I understand how monero works. Yeah you’ll get away with buying some groceries or whatever with it, but people who go through bankruptcy (especially who aren’t rich, and are felons) have a close eye from the government on their finances. If you try to buy a house or a car or anything actually life changing with that, you’re pretty likely to get caught and charged with fraud etc., unless you legitimise it, that is, launder it.
You seem to be a bit confused about how asymmetric encryption works. There is no need for private keys to be transmitted for a messaging service to work. I encourage you to read about the difference between public and private keys in asymmetric encryption. They are generated in pairs, such that when something is encrypted using a public key, it can only be decrypted using the corresponding private key. So it’s not correct to say that the message can’t be decrypted by the intended recipient - they are in fact the only party who can, but even the sender can not.