There seem to be three possible verdicts within the potential outcomes of the Supreme Court hearings.
1 This is a political issue outside the jurisdiction of the justice system which cannot rule on such a political matter.
2 The Court can adjudicate the matter, but rules that the prime minister was and is within his prerogative powers to prorogue parliament to suit his political agenda, within reasonable limits, which were not in this case breached.
3 The Court can adjudicate the matter and rules the prime minister acted improperly and the proroguing is void and of no effect.
Clearly one and two will be victories for Boris. My guess is that two is the most likely. It leaves open the ability of the Supreme Court to be a last resort, if a future government goes down a bad path. It also somewhat redresses the weakening of the Executive in favour of the Legislature, following the FTPA and the SC judgement over Article 50.
The risk is that we end up with a weak executive and a splintered parliament. There are already six parties in the Commons, seven if Sinn Fein took their seats. If the Brexit party win any we would have eight. The system was designed for two. Chaos will loom. So if the decision is number three, it will certainly be a defeat for Boris from which he might not recover. But perversely it could be a defeat for us all.