You’re assuming that unaffiliated voters are not loyal to one of the parties. Very few voters are truly independent. They vote consistently with one or the other. They don’t go back and forth. That group is much smaller, if it exists at all. They talk about swing voters, but really it’s a question of which side’s unreliable voters show up.

Even if none of that was true, you’re still completely discounting the reality of the electoral college and winner take all. We can imagine anything we like, but there is no currently mechanism that would allow for a successful third party run. All they’re going to do is make sure one of the other parties loses. They can’t win.

It’s like playing spoiler.