The other way I look at it is to look at political parties like a business. The business of the RNC and DNC is to sell the product of votes for candidates. Like any business, there are naturally some customers who go back and forth, but if a business suddenly sees 15% or more of their business leave when they are used to 1%, they are going to see why those customers are leaving and adjust their business accordingly to stop losing customers. If third parties (be it Libertarian or Constitution) take a good bite out of the Republican voting block and fundraising (and this also applies to the Greens and DNC this year so it isn't a one sided bleed), it will make the party shift its focus back to the right (hopefully).
One other thing is if the third parties (all of them) can pull enough votes to prevent either major party candidate from getting 270 electoral votes (which is not out of the realm of possibility at all and has happened twice before), you get basically a big red reset button and the decision goes to the House (for the President) and the Senate (for the VP).. .then yo really get complicated but it does allow for a stop and breathe moment to reevaluate the situation based on changing factors (candidate's health, mental state, hacking revelations, foreign involvement revelations, etc)
Maybe there's another way to look at it, too. It's been quite a while since there's been a major party realignment -- maybe as far back as the replacement of the Whig Party by the Republicans. There's obviously been some movement within and between the R's and D's since then, but it's been pretty static in terms of it being a two-party system. The modern political system is essentially built on the existence of those two major parties.
People in general identify as members of one or the other party, and that self-identification is very compelling for reasons that are often more emotional than rational. To convince a person to move between parties is a big deal, but not terribly frightening.
But to move from that stable two-party foundation to a
different party -- that's very disconcerting, and even scary. One must deal with a lot of uncertainty, and grapple with ideas that were not even in play with the two major parties, whose basic tenets have been long-established, if often honored in the breach. It's damned frightening. It took me a long time to work up to saying in public that I would not support the GOP candidate.
Now consider what happens in a year when there are two train-wreck candidates whose very nominations prove that the current parties are irretrievably broken. This makes it easier for people to consider alternative political parties, and perhaps it begins the realignment process into a different set of viable political parties. I've long wondered why a more centrist party hasn't emerged from the rubble that used to be inhabited by rational Democrats and moderate Republicans. Maybe this is the year that realignment begins.
