Even if the justices ultimately rule against Trump, the court’s intervention adds a further delay, meaning his trial will not start for weeks, if not months.
Hypothetically speaking, let's say Biden publicly assassinates Trump. Takes credit for it and everything. Does that mean that the court would be forced to decide on presidential immunity thus fast tracking the decision that presidents are in fact NOT kings. Who cares about the failed usurper when we have a full blown king slayer to punish.
The conservatives on the Supreme Court wouldn't allow a Democrat to get away with this blatantly illegal act. So if we force their hand maybe things play out differently. Presidential immunity should have limits and not allow free reign while in office.
The whole premise of presidential immunity seems purpose built for the next round of fascist/authoritarian candidates. With that in mind, Trump feels like the beta test finding where the road blocks are and taking them down before the real danger starts. Realistically, Trump isn't smart enough to do any real damage by himself. The biggest danger was always the people he surrounded himself with.
Hypothetically speaking, the conservative Supreme Court justices could be included in the official order from Biden... Wouldn't have to worry about the conservative justices making the wrong decision then...