Seriously, what will happen to the GOP 5 years from now when their constituents become too geriatric to vote?

The next generation takes over. This starts happening when Liberals, after getting out of college, start thinking about the political issues instead of just reacting to them. They also start considering the broader foundation on which those issues are based. This includes discovery on their own of the principles of the Founding Fathers, Declaration of Independence, the Federalist Papers and the Constitution.