Trump brought in the bare minimum necessary to scratch out a win. By no means is this enough to even get to 2020. If Trump had won by 10 points or more, I might say, ok, looks like maybe something has changed, but he lost by 2% and barely carried the states he needed to win an EC victory. Further, part of this was based on him having no past record (he will have a tarnished record full of flip-flopping in 2020) and unsustainable turnout among smaller white voter demographics.
Whatever. All the next successful GOP candidate needs to do is keep the rural WWC voters Trump brought into the party while winning back those educated, suburban Romney voters who were turned off in 2016. That is eminently achievable. And keep in mind, you only need 270 electoral votes; Democrats can run up the popular vote in states like California and New York where millennial Hispanics live, but if Republicans hold their gains the Midwest they will keep winning the Presidency. As we saw in 2016.
Let's not forget something else - Trump deviated from the standard GOP agenda in regards to trade and social programs. Does anyone think this is taking the GOP by storm? Even Trump has pretty much abandoned all those promises he made that won over the people he needed to win over. In 2020 he'll be nothing but a guy who made a hell of a lot of promises then blatantly went back on them literally as soon as he was elected. Suffice to say that the next Republican candidate will likely stick to more traditional Republican policies.
Yes, I think that thanks to Trump what it is possible to advocate on trade and social programmes in a GOP primary has shifted towards what is a more winning position in the general, to the benefit of the party.
Anyway, Trump is scandal-ridden and 2020 already looks like it will be a write-off for the GOP, no argument here. But this thread is about the strength of coalitions over the next few cycles, not about how competent an individual candidate is. Trump has proved the path is there: all Republicans need to do is find a candidate with less of his weaknesses who can carry the same message.
In effect, Trump has really changed nothing beyond 2016. If the Republican Party was an addict that was completely broke, Trump would probably be a bag of PCP-laced dope they found on the ground that got them high one last time before they hit rock bottom.
The Republican Party has literally won 3 of the last 4 elections. These metaphors may be comforting, but they are delusional.