The feds are very much into deplatforming people for legal activity, e.g. https://en.wikipedia.org/wiki/Operation_Choke_Point

Generally speaking, most people would consider "not usable in crime" to be a feature rather than a bug. ;)

Marijuana sales, for example, seem extremely popular.

Just because there’s a law in place at some remote level of jurisdiction doesn’t mean it’s just, or has popular support.

Widespread consensus heavily influences prosecutorial discretion though, which determines in practice what is or isn’t permitted by society.

The law is not a computer program, and it is applied entirely arbitrarily.

Your comment makes no sense at all. For something to be a crime it has to be explicitly defined in law. Consensus has zero to do with it. Either there is a law, or there isn't. The whole point of rule of law is that you don't get to pick and choose which law applies to you in a given day of the week.

If the law is not enforced, then it does not exist. The only real law is the one that you see day-to-day. So shooting black people is fine (in the US), and you can buy weed in shops (in a few places).

Also, Prohibition.

The law is not some inviolate thing in a society. Many millions will happily ignore unjust laws if they feel they shouldn’t be laws. More alcohol was consumed during Prohibition in the US than before it was banned.

I don't understand your train of thought. I mean, you are arguing for a monopoly controlling a centralized service just because one of the many freely available payment platforms you personally choosed may have terms of service you might break?

How is that a solution?

It seems you are not arguing for a solution to the problem you framed. You're instead confusing wishful thinking with a solution to the problem. I mean, your suggestions don't even qualify as unintended consequences: they are only unrealistic and absurd expectations from a reform that has absolutely no relation to the problem.