Migo

As a rule, Migo is built on the premise that the claimant could have made another, better claim, and therefore is also loyal to the current, lesser claim.
Therefore, it is clear that one cannot claim a lie if the claimant could not truly claim the other claim, for example if it is a claim that requires courage (and therefore one is not excused from admitting a lie that was a complete liar, because a person does not dare to lie to his debtor).
I asked about the current claim he is making and not about the other claim – here it is ostensibly not supposed to change what he is making and how unfounded/problematic the current claim is, after all, his loyalty is based on the fact that he could have made a different claim. And yet the Gemara says that if the current claim makes the claimant wicked – he will not be loyal even if he has a migo (for example – Ketubot 18: – Our handwriting is this, but we were forced to do so because of money). Why is this so? Why is it relevant that his current claim is not legitimate because he makes himself wicked, after all, loyalty is built on the power to make the other claim that is better than it?