> Your assumption is that Donald Trump forged some kind of political coalition. He didn't.
You are incorrect about my assumption, but correct that Trump did not forge a coalition. The coalition existed before him and largely held its nose because members of it did not have any other options.
My assumption is that both political parties are large, standing broad coalitions of voters with differening interests that are together mainly as a consequence of the two-party system. Which means that they each contain elements that can (and occasionally do) flip party.
Trump exploited a crack in the Republican coalition that had formed over immigration, which was a wedge issue between the "pro-business" type Republicans that are "part of the establishment" and the party base. He also identified other wedges, like military adventurism. That, along with lack of a sufficiently strong opponent in the other 16 people who ran, enabled him to win the GOP primary, even though Trump has very little to no interest in "conservatism."
There were enough Republicans put off by his lack of "conservatism" (e.g. the lack of interest in the agenda of other parts of the GOP coalition) that they were willing to vote for someone else. But because the Democrats had nothing but contempt to offer those voters, they either stayed home, held their nose for Trump anyway, or voted for third party candidates like "Egg McMuffin."
Those divides in the GOP are not going to go away. Those voters are out there, waiting to be picked up by somebody. They may be as much as 20% of that party, which is enough to tip future elections towards the Democrats forever if they play their cards right. But that's going to be an opportunity that isn't going to be exploited as long as the Democrats are playing "resistance" as their strategy.