There's a bit of a hope among political professionals, particularly among those who oppose Donald Trump and what he represents, that there will be a silver lining to a crushing defeat: Republicans will learn from their mistakes and change the party for the better. The problem with this theory is not that Republicans won't learn lessons, but that different Republicans will take much different lessons from a loss. In the end, the same forces that propelled Trump to the nomination are going to prevent the GOP from reforming in the wake of his loss.
The idea that the GOP will change rests on the assumption that everybody will blame the same factors for defeat and that there's a party apparatus in place that could then make sure that the party doesn't repeat those mistakes. Neither of these is true. After a Trump loss, there will be Republicans who say that the GOP needs to be more inclusive, both in terms of rhetoric and policy, and point toward the need for a massive change on issues — immigration being the most notable (liberal Greg Sargent floated this possibility in the context of a potential Trump loss in the red state of Arizona). There will be others who will say that the party needs to focus more on policy, moving away from an emphasis on marginal tax rates toward policies that appeal to working class whites who gravitated toward Trumpism during the primaries. There will be conservatives who argue that Republican leadership is to blame, for sowing the seeds of distrust among the base that fueled Trump's rise. Meanwhile, others will blame conservative outside groups for creating that distrust, and destroying institutions that in previous years would have been a bulwark against Trumpism.
Yet at the same time, there will be people arguing that Trump isn't the problem at all, but that the GOP establishment didn't do enough to help him, and the Never Trump crowd undermined him because they wanted to elect Hillary Clinton. There will also be Trump supporters who will argue that massive, systemic, voter fraud was to blame for Trump's defeat, and they'll want tougher voter ID laws, which will cut against establishment Republican efforts at minority outreach.
This is only a brief summary of some of the divisions that will exist, but an additional problem is that there's no party apparatus that could effectively implement any sort of changes even if they wanted to, because as we've seen, there's a tremendous distrust for the party establishment, and voters won't necessarily go along with what they think is best.
Basically, it's hard to see the post-election period being any different than last time. After Mitt Romney lost in 2012, Republicans had their autopsy, determined they had to reform (especially to attract Hispanic voters), even a shell-shocked Sean frickin' Hannity was clamoring for a path to citizenship. Sen. Marco Rubio was in many ways the personification of what the GOP consultant class wanted in a candidate — young, rhetorically gifted, somebody with a Hispanic background who spoke Spanish, a senator who rolled out a family-friendly policy reform agenda. And his presidential candidacy was a total bust — there was virtually zero constituency for his brand of Republicanism.
Now, to be clear, saying Republicans aren't going to change isn't the same as saying they'll never win another election. After all, there are still only two major parties and sometimes even a weak opposition party can win by default. It's hard for any party to hold the White House for four consecutive terms, as Clinton would have to do to be re-elected. Furthermore, it's very likely that Clinton, who is broadly distrusted and unpopular, will have a troubled and scandal-plagued presidency, leaving her vulnerable in 2020. In the Republican primary, perhaps nobody will be able to reproduce the Trump coalition, paving the way for a more traditional nominee. But the broader forces confronting Republicans are not going to change. The United States has been rapidly shifting demographically, and Republicans are struggling to appeal to the modern general electorate without alienating the older whiter Americans who vote in their primaries.