This will be weird no doubt
If the SCOTUS denies the appeal they will have said that gangbangers, drug dealers and criminal have to be brought back into the US and fed, house and given legal representation all at the cost of the US taxpayers.
This is a bad hill for the Left to fight on. Most Americans will not be happy when this happens and Trump may indeed have the ability to just say...no Fuck you SCOTUS try to enforce your judgment.
If the Dems start screaming all about dictatorship and Hitler, well for one they are already doing that and two they will have to defend bringing back these criminals. That will be a hard push for sure with all the loss of power of the Dem shill machine and Trump's high approval ratings.
I would rather not see this happen, in a perfect world SCOTUS would agree and say this is not the realm of the US judiciary and leave it to Congress. Not that won't happen but I can hope.