Why would such a person suddenly change that attitude if someone else arrives on the scene as Moshiach?
Depends how mashiach is revealed. If Hashem pronounces something unequivocally that contradicts the Rebbe, that’s a reason for a change of attitude.
But the thought experiment is flawed. It’s like asking someone what happens if mashiach doesn’t come by the year 6000. The very question is mufrach, there is no such scenario.