There are massive upsides to US citizenship. Few countries will exert the same effort as the US to help out any of its citizens that end up in harm's way in other countries; Americans have a high level of default business credibility; in many parts of the world, having an American employee can bring considerable prestige and so on. Of course these days it also means being more of a target for terrorism and so on so it's not all gravy for sure.

If you have already formally renounced US citizenship, though, getting a visa or a green card is damn near impossible. You'll have to put a lot of money on the table to win that argument. The rules very strongly discourage deals of this kind, though that probably doesn't mean much any more if the last 3 weeks are a guide.