The opinion is not nearly as broad as certain sources say it is. What it actually says is that immunity as to "core" powers is absolute, but it doesn't define the scope of those powers. It says other official powers have some immunity, but are not immune basically if allowing the prosecution wouldn't impair future presidents. Those are both exceptions that are larger than the rule.
Some are saying it is broader than it is because they want charges to be dismissed. Others are saying it's broader because they want a more powerful executive. Others are saying it try to make the court look bad, or to get votes. Some are saying it because they're mistaken / confused. On 4 out of 5 counts, the court remanded for further proceedings without deciding whether Trump has immunity. All of these voices could have easily come out with a different take, claiming victory and saying he will likely be convicted on most counts.
Totally agree. Look at the Supreme Court nominations under Trump. He hit the jackpot. But then there was a bunch of talk by the other side of adding justices. Do we really want to open that can of worms? That's a blade that will cut both ways once the precedent is set.
Same question applies to either party when they push stuff like this.