Because demographics change? Right now those major cities may be liberal leaning but that isn't set in stone. Populism, environmental factors, media, trends etc etc can all change the political leanings of a generation. For example, the surge in popularity of right wing populist candidates globally. As for Libertarianism... that's just a bad political system (no offense to you). It completely fails to take into account things like basic human nature.
Oh, I don't think all of anywhere's problems will ever be fixed. Perfection is an impossibility. You just need to fix enough of a places problems that it will function as fairly and justly as possible for as many people as possible. That's not completely beyond the realm of possibility in 4-8 years. I will say though, you did just have a good president. I totally disagree with his war-related policies, but Obama was pushing for a lot of positive change in the US. He was just cockblocked at every turn by obstructionist republicans and vilified by the media for no good reason. I don't really know enough about JFK to comment on him.
Probably, but they get payed shitloads already (too much), so it stands to reason most wouldn't risk the large amount of money they already have to give it a slight boost illegally. The vast majority of that tax money, I am pretty certain, does exactly what it's meant to.