Without Trump’s base (primarily working class, disproportionately of European descent) the Republicans have a problem. Even if you add up the “Tea Party” and the “Neocons” and the “Country Club Republicans” and the “Wall Street Republicans”, they don’t have a majority. Bush only got in since the earlier version of today’s “Progressive Democrats” stabbed Gore in the back and voted for Nader. Reagan only got in since Jimmy Carter was a clear failure and the Democrats in the 1970s were pushing a radical agenda very similar to today’s “progressives”.
Unless they can convince the Democrats to run a socialist or a fool, the Republican best chance to win is to find a way to unite their factions in a way that includes Trump’s base. If winning was more important than ego, Trump would settle down to be a “Senior Statesman”, and become a good party man (instead of someone who was recently a Democrat, and always focused on himself rather than party). Getting Trump to settle down to retirement as a “wise elder” is a challenge.