I'm not so sure about that.The GOP is going into the garbage pooper of history a lot faster and that as soon as Dallas and Houston and SA and Austin blue up enough to swamp rural TX and that massive mega-state sails off to join CO, AZ and VA in the blue camp forever and ever. The GOP can't kill off its moderates and go ultra hard right without provoking a backlash to which it will eventually lose as younger voters, urban and suburban voters and more educated voters - all major demographic trends - kill it off. If the GOP is going to force feed anti abortion, pro gun judges on an America which is sick of that shit, then the backlash and the Dems stacking the court w moderates is going to be a popular move.
Before Trump, I would have agreed with you. The GOP was essentially moribund. I thought that with Obama that it proved that demographics were simply not on the side of the GOP and that the Democrats would become the natural ruling party of the USA (to borrow a phrase).
But then 2 really strange things happened.
1. The Dems picked Hillary as their candidate. I mean holy fuck, could they have picked a worse possible human being than Hillary. I know everyone has baggage that they are dragging around behind them, but Hillary? Hillary is dragging a statue.
2. The GOP picked Trump. At first, I thought the GOP were out of their collective minds. But the madman won. Not only did he win, he won big. (Yes, yes, I know Hillary got X number of million more votes than the orange clown, but so what? That's not how the American system works. Based on the quaint concept of an electoral college, Trump won large. And that electoral college isn't going anywhere. You know it and I know it. Just so happens that there are a lot of liberal voters in California, but it makes no difference if you win by 1 vote, or all the votes, you win, you get all the electoral college votes, end of story.)
And here's the weird thing. Trump really isn't a Conservative is he now. Hell, he was a Democrat for most of his life. Not quite Tammany Hall Democrat, but still a Democrat. Definitely NOT a Republican. I would argue that Trump doesn't actually have an ideological bone in his body. He moves whichever way and wherever best suits him to achieve his desired outcome. He is at his core, a populist. Simple as that. Trump is not really in heart of hearts anti-abortion, nor is he a gun rights fanatic. But he figures (correctly) that he will use it to his advantage, but then does nothing to advance the cause. Nope, Trump pushes that populist mantra that appeals to a great many people in the USA. Young or old, dumb or not so dumb. America First, Make America Great Again, blah blah blah. Americans love a guy who they feel makes them stand tall. (Same as Reagan. Carter was a better man than Reagan, and so was Michael Dukakis and yet Reagan slaughtered both of them. Why, because Americans love what he was selling them. Dukakis was the champion of the working man, but the working man rejected him in favour of John Wayne.) Just like now they picked Trump and his "America First" rhetoric. He sells them what they want to hear from a politician and never have before.
"Trumpism" has given the GOP a new idea. It's brought out the whack jobs who probably have never even voted in the past, but suddenly, they've found a champion in Trump. Combine that with the your dyed in the wool Republican types (i.e. the ones who aren't fucking crazy) and you've got a recipe for a win. I expect the next leader of the GOP after Trump to seize on what worked so well for Trump (populism) and just be sane at the same time. (Something Trump is not.) In other words, Trump may fade away, but Trumpism will be here for a generation because the old Reagan / Bush styled Republicanism will not win you an election given everything you've said in your post.