The problem with Trump being found to have immunity is that it means that Biden has immunity.
He could then arrest Trump and the corrupt conservative in SCOTUS and install his own liberal justices without breaking the law because immunity.
Republicans MIGHT want to avoid this scenario.
Trump doesn’t have the power of government behind him. A Democrat does.
The reality is that Republicans can’t depend on Democrats playing ethically (see Senator Robert Menendez).
Okay, so Biden decides that the best way to solve the problem is to execute SCOTUS (this is all hypothetical) — no problem as he has immunity. As such, he could, again in theory, grant others immunity (because nothing says he can’t).
This is the problem with putting people above the law — it allows them the ability to change the law as they want. Because it isn’t illegal.