The Establishment and the media hate the guy. He must be doing something right.
He's an orange 6'3" monkey wrench we can throw into the bloated self-serving DC cronyism machine. I hope he's a patriot and will at least attempt, if not succeed, at cutting taxes and regulations.
What is there to lose with a Trump presidency? nothing, really. Both sides of the aisle will will be so devoted to maintaining the status quo, shutting down the Trump. Worst case it will be a contentious and exciting 4yrs, and nothing will get done. The country will be fine. pssst, this is one guy, folks, the president is not a king. He can't unilaterally do this, end that, nuke anyone. Sounds pretty good.
What is there to gain with a Trump presidency? lower taxes, reduced regulation, secure borders. He has a proven 40yr history of running his mouth and then making smart, risk-averse, considered decisions. Sounds even better.
I think many anti-Trumpers don't like the fact that he's a giant prick. Sure he is. Many New Yorkers are (sorry, but true). On a personal level I despise the guy myself. But that is irrelevant.
Trump is a tool. He's a once in a blue moon, maybe once in a lifetime opportunity to disrupt the Establishment and cause real change.
You do a great job of pointing out why, if elected, he would be an utter failure. Mostly due to the fact that both sides of the isle will bring a full stop to his asinine, and delusional bluster, and he would accomplish nothing. Then you mention the up side of lower taxes, less regulation and secure borders. I'm not sure how you can reconcile being aware enough to understand that he would be nothing but an immediate and giant failure, and then speak of all he will accomplish? Finally, his "greatness" is largely the babbling of a deeply disturbed egomaniac. IIRC, he has had a long running feud with Forbes, which frequently states that he is worth less than half of what he claims, and often gives bat shit crazy valuations to his holdings, to bolster his net worth and fragile ego. It has also been stated that had he just taken all the money that his daddy gave him, and invested it in a low cost index fund, he would be worth well over twice what he is today. So basically, he is roughly half as successful as our overall economy is, long term.
Given the well documented history of everything from racism, stealing from contractors on his trophy projects, running his publicly traded company into the ground, while bleeding it dry, and transferring personal liabilities into it, and countless other examples of living in a total moral and ethical vacuum, it's amazing to think that anybody would consider him to be worth of any office. Last, the whole concept of, "oh, what's the harm of having an interrupting force like Trump in office for a term, it's not like he can really do any harm, anyway?" can only be described as stunningly ignorant. Electing a narcissistic, egomaniac who has clearly self-identified as being totally unit for the job, to the most important elected position on the planet, is a hell of a lot more serious than, "Oh well, what can go wrong, it's not like he is a king, or something?"