The financial crisis killed them. There just aren't that many social conservatives with liberal (in the economic sense) views any more in America. That's the reason why Romney lost handily in 2012, and why so much of the Republican base backed Trump over the more 'traditional' candidates in 2016.
This is correct. Capitalism is great, but some regulations are needed and the common man and woman on both sides of the aisle wants a government that puts their interests before those of billionaires and their multinational corporations. The media has shifted the country to the left socially to the extent that gay marriage, just a decade ago a winning issue for the GOP) is a done deal with very few seriously still trying to reverse it. Reagan's coalition didn't really believe in all the "true conservative" principals of Goldwater anyway and neither do the modern ones. Goldwater voted down the Civil Rights Act as an unconstitutional federal overreach. Guaranteed nobody important in the GOP would stand on principle enough to agree with him there (meaning you yourself are liberal by the standards of the past). The conservative movement has played a losing game for the last 30 years and failed to conserve anything. You already lost the culture war and if you don't do something to slow down demographics the whole country will vote like California.
Trumpism is still unrefined, but that gives it the potential to develop in whatever direction it needs to go. Most importantly, it replaced the cowardly defensiveness of conservatism inc. with an attack. With something to vote for, not just "vote against socialism." If "true conservatives" had conserved the America of the 80s, then Trumpism never would have been necessary. What happens is that you hop on the Trump train and try to keep it as far to the right as possible (while accepting that the borderline theocratic social views of the Roy Moores of the world are dead) or else you become irrelevant.
Even Ted Cruz had to adopt populist views to be able to run to Trump's right. Ultimately, the Cruz and Trump voters have enough in common that they should support each other. If you side with the establishment that has lost and/or betrayed its base so many times (amnesty bills, failing to repeal Obamacare, allowing the debt to skyrocket, etc) over Trump, I don't know what to tell you but this: Expect to continue losing
P.S. Pence is the favorite to succeed Trump and should unite the GOP behind him. Just wait.