> On top of this there are 'tumblers' that do this as a service.

and using these makes no sense, since it's readily noticeable that you've used one, and using them is basically always illegal. The crypto currency ecosystem keeps reminding me of https://xkcd.com/1494/ .

This is incorrect. Things are legal unless criminalized, and tumblers are not inherently illegal, and have plentiful legitimate uses. However, money laundering or offering a money laundering service is illegal and so using one in an effort to money launder, or facilitate such, would be illegal. It's akin to something like a jimmy bar. Owning or manufacturing/selling them is 100% legal, but they're going to be used pretty regularly for illegal purposes which is, rather tautologically, illegal. Similarly, intentionally selling them for illegal purposes would be illegal.