That doesn't really play out when you look at past election cycles though. Sure, Trump is a wild card, but the Republican primaries always go waaaay far to the right early on, and swing back to the center by the convention. I don't see Trump getting enough delegates to win outright, which most likely means a brokered convention. If that happens, I'm fairly certain the party will do whatever it takes to make sure he doesn't get the nomination since he has no chance of winning the general. He's only winning now because of far-right grassroots support that a lot of similar candidates enjoy early on, but that support always fails to materialize come November.
I also don't see Hillary or Bernie having any trouble at all winning the election. There are way more paths for them to get to 270 than for the Republicans. As it is, Dems only need to win 1-2 of the swing states to win, and Bernie and Hillary are polling ahead of all republicans (even Trump) in all the states they need to hold, as well as more than enough of the swing states.