Why should Assad go?
Why should he stay?
He's a parasite, whose regime was so busy gorging itself up that it didn't do anything when the worst drought in 900 years wrecked his country. His dynasty's survival from this conflict would only insure that ten, twenty, thirty down the line another uprising begins, one that will likely be even more violent from the get go, and faced by a regime that will only have grown more decadent and inept that it already is, because that's the thing with dictatorships: they never become more competent as time goes by.
To be blunt, the longer Assad stays, the likelier the Alawis, Christians, and other minorities which in exchange from preferential access to the scraps became the de facto hostages of the regime will end on the receiving end of a genocide fueled by revanchist fury when (not if) his regime collapses.
Trump doesn’t understand that the United States is not a nation-state. It would be, if it were a state bound to an ethnic or linguistic identity
Actually, Nations are not bound to ethnic or linguistic identities. The modern concept of the Nation was conceived in opposition to regimes ruled by tiny clades of hereditary aristocrats: at its core lies the idea that the subjects of any given crown form a community of interest that supersedes any other identities. You may not like your compatriots, but more often than not, your prosperity will be more intrinsically intertwined with theirs than with people who share your religion, ideology, or esthetical preferences from the other side of the world.
What Trump is doing is repeating an imposture propagated by european far-rightists: what they do is childishly simple: they take the herrenvolk regimes they want to establish, and call these "Nation States": these regimes are as much "national" as the nomenklatura-ruled soviet-style dictatorships were "popular", but sheer repetition is often sufficient to spread confusion about the far-right's goal.
If anything, Trump is simply another manifestation of something that has been taboo for many years: many white westerners hate the "Western Values" and the type of governments they spawned and would rather abolish them and come back to the pre-enlightenment despotic regimes where civilization itself existed only to cater to the whims and demands of a tiny parasitic aristocracy while the rest of society lived short and miserable lives toiling in the mud to sustain their lords and masters' material comforts.
Trump's supporters, like the supporter of european far-right parties are not dupe about the actual goals of their champion: their folly lies in the fact that they are convinced that they will belong to the new aristocracy Trump wants to establish: they won't, and most of them won't even get the lackey's preferential access to the nobility's scraps.