In general, the FBI doesn't let criminals just slide when the scumbags steal a bunch of money and then launder it in the open if they have enough hard evidence to convict them.
But "we take every amount of money of which we can prove beyond doubt that it was illegitimately acquired" is like trying to deal with theft by forcing the thief to give back the goods when caught and nothing more.
Humans are creative; at that point money will be laundered through various overbilled (or even outright fabricated) medical procedures, paid for in cash.
We frequently don't catch murderers; this does not mean we should not investigate new methods of catching them.
You really think they couldnt think of anything lucrative over the last 30 years with completely new and separate money?
People that understand capital markets dont just have 1 lucrative idea their whole life.
The 2014 service providing in St Kitts and Nevis and parlaying that into Bitcoin and Ether would easily have earned them billions by now on its own. So it is impossible to assume anything.