The ascendance of grassroots conservatives within the party couldn't really be curbed, but they might not have shed the Tea Party moniker in favor of this bizarre Trump personality cult. Really, the amount of influence Trump has depends on how big his loss is, and I'm guessing you're talking about the 278 map for Hillary. In any case, I could see a Rubio win in 2020 predicated on lockdown backlash. Increasingly narrow margins of victory for the Democrats over those three consecutive wins would also indicate some disillusionment with the party, and Trump might have been remembered as "almost there, but not quite".
I really don't think it mattered how badly he lost by. It was extremely apparent that they were looking for any excuse to get rid of him, and even if he came close they would just say that anyone else would have beaten Hillary.
Again, just think of how many Republicans unendorsed him after Access Hollywood. Think of how many who are now sycophants denounced him even BEFORE the election, let alone before a loss. It's obvious they expected him to lose and that they were preparing to make a "Told you so" pivot after he lost in an effort to be rid of him for good.
Whether this would work on the base or not might be a different story, but I imagine that a loss would have demoralized many of his supporters too. Undoubtedly he would have still claimed it was rigged, fraud, etc. but attempts to fight the results would not have gotten nearly as much support among other Republicans as they did after 2020.
Probably they do another "autopsy" and take steps to ensure the nomination can't be taken by another Trump next time. Remember that Trump only ever won a plurality in the primaries in the first place; if they had coalesced around a more mainstream figure from the start rather than have like 5 guys with overlapping bases drag their campaigns out for too long, Trump probably would never have won the nomination. So they'd probably be sure to do that in the future, among other things. No, the grassroots right-wingers wouldn't go away, but I do think the establishment would want to contain them as much as possible even as they might still throw them a few bones. But they would REALLY want to enact the reforms they proposed post-2012 to have more general appeal after a third loss in a row, and to a woman they'd demonized for decades in preparation for her run of all people.
