Political fortunes come and go. Both American parties have had good times and bad times in the past and I doubt that under the system in place, that that will change.
It's isn't unique to the U.S. either. In Canada, in 1993, the Progressive Conservatives went from a majority government of ten years with 151 seats in the House, to 2 seats. Everyone said that they were finished and that after Mulroney, they deserved it. Guess what? They recombined with the western splinter Reform party and now form the Government of Canada again. In the UK, the Labour Party was flying high ten years ago and look at them now. They are in disarray and will likely go down in the next election.
I think predicting the death of the Republicans is foolish and has little grounding in history. They could reform or change their name, but in the end you'd still have a left wing party and a right wing party, both competing for the centre. But it's interesting reading and I suppose it does sell papers.