This most recent ruling wildly expanded the immunity, added presumed immunity for adjacent actions, and phrased everything in such a way that actually prosecuting the president for literally anything will take years.
Say the president does something you think is illegal and should be prosecuted. Stop. Before you can take him to court over that, you need to determine if what he did was "official" or "unofficial." SCOTUS didn't give deterministic guidelines to differentiate, so you need to have a separate court case just for that. Alright so let's have the court case that determines whether what the president did was official or unofficial. Let's introduce some evidence—
Stop. Evidence from official acts cannot be introduced in a case to prove something was unofficial. So you actually need to have a separate court case to determine if that evidence is official or unofficial. Once you have your results, one party won't like it and will appeal it up and up to the supreme court. Repeat for potentially every single piece of evidence.
Okay now that we know what evidence we can and can't introduce, we can finally determine if what the president did was official or unofficial. Once we have a result, one party won't like it and it will be appealed all the way up to the supreme court again. Only when SCOTUS rules the action was unofficial (IF they rule it was unofficial) can you then BEGIN the process of actually taking the president to court over that action.
This will take years, not to mention the supreme court is appointed by the president and it recently ruled that taking bribes after you do something instead of before is perfectly legal actually. This is all by design. The point is to keep this all tied up in court for years, which effectively gives the president full immunity for everything. And he can also pressure the courts or judges to rule his way via any number of threats (if you think that's an unofficial act, feel free to take him to court over it).
This is pretty clearly designed to functionally protect the president from all culpability (which the dissenting SCOTUS opinions agree on, ergo their dissent).