We can conscript as many as we want if we pay them enough. If we're willing to draft people, then why wouldn't we be willing to raise taxes?

Taxpayers are generally better organized politically than potential conscripts.

By the second point, do you literally mean it's legal to conscript soldiers (it is in America at least, although starting a draft would be politically impossible absent an immediate existential threat to America as a state), or do you mean that figuratively, in that if we pay soldiers enough, we'll get more volunteers? I'm not sure what point you're making.

I will see if I can find the data on the poor performance and high cost of mercenaries.