Both the possibility that corporations may not come to the US market, or may only come to the US market after having expanded into other, more-profitable markets require an assumption that the US would (substantially) raise taxes above the current level.

We're hypothetically setting out to close a loophole that results in two distinct effective rates, a lower rate A for foreign corporations and a higher rate B for US-based corporations.

So, again, why should we spend any time worrying about what might happen if we were to introduce an even higher rate C? No-one's talking about that. It's not on the table. It's not remotely politically plausible. It's not even a thing anyone's seriously proposed.

Talking about it is either entirely besides the point, or a motivated attempt to conflate closing the loophole with raising rates to C, to scare people away from trying to close the loophole.