The problem I've encountered with schemes like you're describing (such as encryption schemes with decoys) is that it also makes it impossible to verify that you've given everything up.
Say you give the attacker all of your bitcoins/data, they're now incentivized to continue punishing you in perpetuity regardless, since you could have provided a decoy.
> I mean: unless the attacker knows exactly how much coins you have,
They don't need to know the exact number. They need to have a rough idea of the number. In the most likely scenario, where a state wants access, they tend to have a pretty good idea, because they get that information from investors, state agencies, banks and seized records.
Well, it was back when crypto still colloquially meant cryptography, not cryptocurrency. Of course, in this case it's the same thing because cryptocurrency uses cryptography underneath.
I mean: unless the attacker knows exactly how much coins you have, he has zero way to way if you're unlocking a decoy wallet or the real one.