Not bloody likely. The GOP is still doing quite well on the state government level, and it is very likely that they will still hold the House after the 2016 election. The Republican Party is 160 years old in a country with a political system that highly rewards a two-party structure.
However, they need to make some changes if they want to win Presidential elections. The United States' two major parties have shown great resilience and the ability to adapt when they're out of power. The GOP has not been successful in doing this over the last several years, but I have to imagine they will eventually.
I don't think the point is about them winning or losing Presidential elections. Even if Trump wins, this is still terrible for the GOP. In fact, a Trump victory may be worse. There is no party unity. Half the party hates the man. Most of the party hated his alternative, Cruz. They were incapable of putting forth a candidate to unify the base. They are no longer a five-ring circus; they are five separate circus tents.
What happens when Trump takes office and has to work with a GOP Congressional leadership? How exactly is that going to go, when he is bound to alienate half of them?
This party is literally falling apart. How do they move forward and pick up the pieces?