First off, when I say "self-destruct" I obviously don't mean that current Republican voters will all evaporate or become liberals. What I mean is that the coalition making up the Republican party will become so fragmented, while its various groups continue to decline as a percentage of the population, that it ceases to be an effective political force. A post-election explosion of recriminations between pro-Trump and anti-Trump Republicans, with each group trying to "purge" the other out of the party (as they are already threatening to do), will sharpen other conflicts which don't map onto the divide over Trump in any simple way. The libertarians have a lot of differences with the flaming religious nutballs over issues like gays and marijuana. The Deliverance-mutant Confederate-flag racists and the wealthier suburban element that votes Republican as a pocketbook issue don't much like each other either. There's some overlap among these groups, but the conflicts are real. Historically they've stuck together because each group knows it can't win national elections without the others. What's changed with the Trump fiasco is that some groups are starting to define others as a liability they can no longer afford, because keeping them within the tent does more harm than good to the goal of winning.
It's easy to imagine the libertarian Republicans decamping to a Libertarian party that shifted more moderate to accommodate them, and/or the fundies shifting to the Constitution party or trying to set up a new party, if either or both of them lost the battle for control of the Republican party itself. I don't know exactly what configuration will emerge. What I see is a breakdown into several mutually-hostile groups (one of which will be the much-shrunken original Republican party), each one too small to compete with the Democrats nationally, perhaps in some cases becoming regional parties -- the fundies in the South, say, and the Libertarians in the inland west. (They won't, of course, be intending to produce this result. Each subgroup will imagine that it has the chance of becoming a new major party if it can just break free of the others.) If this scenario happens, then in national politics the US will temporarily become a one-party state.
I say "temporarily" because, as we well know, there is a major division within the Democratic party as well, one which our recent primary brought sharply into view. The party has what might be called the centrist, pro-business wing exemplified by the Clintons and Obama, and a more explicitly socialist, redistributionist wing represented by people like Sanders and Warren. The two are held together, for now, by fear of the deranged Republicans -- polls show most Bernie supporters voting for Hillary, however reluctantly, and the same unification would have happened if Bernie had won the nomination.
But if the Republican party breaks up and is no longer a threat, I could see these two wings pulling apart and eventually becoming two separate parties -- call them the Centrists and Socialists (though obviously those won't be the actual names) -- and thus restoring a two-party system. The Centrists would likely absorb some of the saner Republican voters and donors -- those who want a business-friendly environment but aren't very interested in bashing gays and abortion or in the ideological-purist craziness of the big-L Libertarians. The Socialists, if they play it right, could attract some of the angry left-behind white voters now enthralled with Trump -- those of them that genuinely aren't racist, at least.
A two-party system like this would actually make the US more "normal", more like most other Western democracies. The Centrists and Socialists would look fairly similar to the UK's Conservative and Labour parties, for example. The UK also has medium-size single-issue or regional parties such as the UKIP (anti-EU) and the Scottish Nationalists, smaller than the big two but not utter minnows as the present US third parties are -- in a post-Republican US, the various fragments of the former Republican party would have a similar role. The situation in France and Germany is like the UK. The US is the odd one out in having one of its two major parties dominated by religious fundamentalists and in denial about basic science.
The scenario I've described would also restore the "normal" system we ourselves had before the Republican party went completely nuts (whenever you consider that to have been). We would once again have two parties a sane person could consider voting for, and not feel too bad if the opposing party won -- as opposed to the present situation where every election is a desperate panicky struggle to keep the gibbering lunatics away from as many of the levers of power as possible, and the details of the Democratic alternative hardly matter.
A blog called The Big Empty recently envisaged a somewhat similar scenario. No one can foresee the future in detail, but it's most probable that something like this is going to happen. I can't see the current situation lasting indefinitely, and the current Republican party is probably beyond saving. Trump just gave a face and a voice to the madness that has been roiling for years.