Why is it assumed that if Trump runs independent that he will only draw the independent vote away from the republican candidate?

If this was indeed a valid assumption why is it not just as viable for the race to still be viewed as Trump or whoever the democratic candidate is?

Is it a foregone conclusion that if Trump goes 3rd party the republicans would knowingly dilute the vote to allow a democratic win?

I don't think so. We are not all lemmings or pied piper voters!!!!