While this is true, you are incurring a non-trivial amount of overhead to track this. Ideally you need to "segregate" the tax portion of those gains in order to pay taxes with....inadvertently spending them will lead to further issues. Then that fiat is going to sit unproductive for months waiting to pay the tax with it.

You could make the argument to just keep the tax gains itself in BTC and liquidate that amount later to pay those gains. That can work, but timing can be a bitch. Imagine you go thru a 50% bear market and now you need to sell to pay for gains you made 8 months prior.

There are lots and lots of day traders that go bankrupt from these scenarios.