Thing is, research organizations have and do survey the public about this. A LOT of pollsters were surveying the public about RFK during this round.
A non-zero amount of 3rd party voters always say they’ll move toward Trump or Harris depending which 3rd party option falls off the pick list.
And when races are tight enough to be decided by a few hundred or a few thousand votes, a small non-zero amount of people can be the difference.
I’d argue that you don’t need it in every state. You just need it in enough states to make a 3rd party candidate viable.