Its not so much immunity as it is arguing that the president can only be impeached by the House and tried by the Senate for his official acts as president - as set forth in the US Constitution. So he’s not “immune”. There is a mechanism to try any president.
The alternative is that any one of America’s approximately 30,000 DA’s including highly partisan ones in highly partisan areas can charge a president. Given the highly partisan districts some DA’s might come from, this is a free license for any district to hold an elected president hostage. ie “enact this policy we don’t like, and we will charge you with a crime and you’ll get convicted by a jury here”.
We simply can’t have that if we are to have a functioning political system.
And then there is the fact that Trump was already impeached by the house and acquitted by the Senate for the Jan 6 mostly peaceful protest so charging him with it in a criminal trial would be double jeopardy which violates the 5th amendment.
Exactly, and I believe enough justices realize that.