It's possible because the other side is no less of a goat rodeo.
Well, yes and no.
The POTENTIAL problem for the Left is for them to commit McGovern-Mondale fratricide by nominating a "true believer Socialist" while at the same time the GOP cuts Trump loose and finds a way to NOT renominate him. The nominating wing of both parties is dominated by the extremists in both directions. In 2020, no Democrat starts out with the high name recognition that Hillary had as far back as 2000, Edwards and Obama had in 2008 with the exception of three folks in their 70s (in 2020) who aren't running: Warren, Biden, and Sanders. The REAL danger for the Democrats is that you might have the Republican problem of 2016 - if too many people think beating Trump is going to be an easy mark ("after all, he only won by 70,000 votes in three states" will be the mantra) then you might have 10-12 candidates, none of them well-known, who split the vote early on and default the nomination to someone who has gained big name recognition for the wrong reasons. The Democrats, of course, have the built-in super delegate votes so they're by no means bound by the primary results as the GOP is. I'm not even predicting - what I'm saying is that it wouldn't surprise me if they made the mistake of thinking: a) Obama got elected BECAUSE he was a liberal (not true); and b) HRC lost because she wasn't liberal enough (also untrue) - and wind up firing up the modern version of McGovern-Mondale, the candidate of special interests. Of course, they won't lose quite so badly as those times because some states have shifted and there is no Ronald Reagan out there for the GOP, either.
There's no telling how big - or how small - a disaster Trump can be seen as. The right was grumbling so much about Reagan that two days before he was shot, he brought down the house at the Gridiron Club dinner by giving the one-liner, "Sometimes our right hand doesn't know what our far right-hand is doing." At that point Reagan had only been in office about ten weeks and it was the right mad because he wasn't dismantling the government (e.g. "draining the swamp"). Of course, Reagan had also been governor of a large state for eight years and knew there was no reason to propose politically risky stuff that had no chance of passing (though this didn't stop him from bungling Social Security reform that May). Bill Clinton was angering everyone by this stage of his Presidency: liberals, conservatives, AND moderates, plummeting 33 points in the polls from January to May. By March 18, 1993, Clinton had abandoned "gays in the military", abandoned his tax cut, had the Coast Guard rounding up Haitian migrants after saying it was okay for them to come here, presided over the first WTC bombing, and had a standoff going on in Waco at the Branch Davidian compound. (And no, folks, I'm not BLAMING Clinton for the last two things, they were crises he had to manage).
Both were goners by January starting the third year (1983 and 1995). And both won easy re-election. Of course, Trump is not like either man. Both were long-term governors who knew that when you threw red meat to the base of your party, you barely acted upon it save for what would be easy and not damage you. And both had manners in how they treated folks (I've said before that I knew one of Bill's limo drivers and a guy on the White House physician staff in 1998 during the Lewinsky scandal. BOTH had much negative to say about HRC's manners towards those serving her but both were completely positive about how kind Bill was to those who worked for him, particularly when you'd think the pending impeachment would take the edge off him).
Trump is 70 and has never had to be polite to anyone, including folks loaning him money. What's funny is if you watch old 1980s videos of the guy, the look on his face and his willingness to play the PR game suggests a guy who actually might make a decent candidate - prior to his letting crassness loose on reality TV. And let's be honest: a good chunk of his vote would have gone to another Democrat if that Democrat wasn't named Hillary.
My point, which I've gone too far around, is that both Reagan and Clinton had been playing the game a long time and had made adjustments as necessary to make their messages more appealing. Trump ran one election and literally didn't change anything at all. It's one thing to have a single election-year exercise in crass behavior but it's also the thing that wears out VERY quickly on TV nightly.
The other two showed they could change and be flexible - Trump never did that, not with the USFL, not with how he spent money, not with how he changed wives, and not with how he ran for President. Hence, I have no reason to think he will change now.
Btw - on a related note - shouldn't these Presidential candidates be picking VP mates that can carry on after them? It used to be the norm but you're not going to tell me that Bush thought Quayle could be President in 1988, that Bush 43 thought Cheney would actually run to replace him, that McCain actually thought Palin could run in 2016, or that Obama REALLY thought Biden was going to run in 2016. Picking Cheney made some sense in trying to establish rapport with the press corps in DC (he had been Ford's chief of staff, a rep, and SECDEF and had high marks from the press before being chosen) and Biden was chosen mostly because he wasn't Hillary and did have a decent TV presence......the other two were disasters but you cannot tell me that any of those choices were made with the thought of "this person will succeed me in eight years." Pence was chosen more as a sop to the Religious Right than anything else...and who knows how many besides Kasich turned Trump down? Pence showed in the debate he has the potential to be a formidable candidate (and a governor's background once again), but he was chosen more by POE than anything else.
And does anyone think for a second that dull as dishwater Kaine is going to make a splash in 2020? I mean, come on.