You're taking the different definitions of money laundering and switching between them as it suits you.
If by 'money laundering' you simply mean transacting in a way that is mostly anonymous, you can do this today just be using cash, finding someone else with cash, mixing your bills together, and none of that is either wrong or illegal.
If by money laundering you mean committing a criminal act and then hiding its financial traces, then that is only one possible use of tumblers, but there are other legitimate uses.
It's kind of confusing that you consider switching bills around to be a normal part of transacting anonymously with cash. I mean, make change if you need to, but this is usually not motivated by anonymity.
It's sufficiently anonymous for most people to just spend the cash they have. You didn't get the cash from D. B. Cooper, right? So nobody's watching the serial numbers. You just spend it.
It's only Bitcoin where that isn't an option.
Edit to be clearer about what my point is: cash has a moderate amount of anonymity, with practical limits. If those practical limits are a problem for you, you probably are covering up a crime. Bitcoin, however, has no level of anonymity between "everyone can see everything you spent" and "covering up a crime".
If by 'money laundering' you simply mean transacting in a way that is mostly anonymous, you can do this today just be using cash, finding someone else with cash, mixing your bills together, and none of that is either wrong or illegal.
If by money laundering you mean committing a criminal act and then hiding its financial traces, then that is only one possible use of tumblers, but there are other legitimate uses.