I'm no legal scholar, but I did stay at a Holiday Inn last night....
IF - he has no legal standing to be president, then I'd imagine that all of his executive orders would be vacated immediately. Any bills he's signed into law would be either null and void, or subject to immediate challenge. Would we just get a national "never mind" and forget about the past few years? I dunno. I *do* know that the stock markets would be rocked - lots of companies leaning one way (think "got to get ready for Health Care", oops, never mind....) would need change their plans 180 degrees. That'd be expensive, to say the least.
I don't know how it would affect our standing internationally, but I can't imagine that it would be positive.
And that's just the legal side. Domestically, I think that if he was tossed due to some sort of irregularity, then I think that his supporters - blacks especially - would riot. Would make LA and Rodney King look like Romper Room.
Besides, that'd leave us with President Biden. I'm not sure that's an improvement.