Essentially the opinion states that the decision is both legal and fair.
“Prerogative and preference” is the core argument for the appointment. That McConnell exercised his power in 2016 is moot because he did what the Senate rules allow.
Fair is in the eyes of the partisans. The regime ran on many things: a Mexican paid for wall, law and order, the “best healthcare ever”, infrastructure, and anti Roe v Wade judges, redoing “horrible“ trade deals, economic boost from tax cuts. Maybe one success is enough to get votes.
The question I have is whether the Senate has too much power as outlined in the Constitution. The national legal framework allows Congress to set its own rules, so until such changes, probably ill advised, are codified by amendments, McConnell is acting legally.
The Constitution was a compromise with both the electoral college and the Senate giving immense power to the sparsely populated states. Not a balance, but a huge advantage. And with the advice and consent clause, the power carries over to the judiciary with lifetime appointments.
When we look at the tremendous struggle to end gerrymandering in VA we see the inherent difficulty of removing power even when abused.