I guess, global seized currency? because US didn't buy Bitcoin as their reserve, but it's from confiscating other entities coins.

It's very very long to happen if we compare with the US reserve.

M2 money supply is $22 Trillion , but the amount belong to IMF as global reserve is $6.7 Trillion

On the other hand, Bitcoin belong to whole countries are only worth $57 Billion.

We will likely to see Bitcoin to be top world asset first, than top reserved currency.