The argument is not so much that mail-ins are illegal, more the specific circumstances they were handled in PA and other disputed states were illegal because it allowed too high a false positive rate. You'd never be able to pull this off in a pure blue state like CA or NY.
All Giuliani and co need to do is to make the case that fuckery flipped places like PA that were heading to Trump before the fuckery went into overdrive.
I think the Democrat fondness for mail-ins will disappear after this. Then again COVID will be long gone by 2024 no matter what happens. In fact given
Cuomo's tweet it will be long gone by mid-2021 according to the media. Mail-ins were always a way for the Democrats to commit fraud in this specific election. Next time they'll try something else.