How important can this actually be to you?
I am all for checks and balances and I have not gotten into the details of why Garland is refusing, but let's not pretend you are not ok with your side skirting the checks and balances. I agree it's a slippery slope America has been slipping on for a while.
How many times was executive privilege claimed when came when it came to congressional subpoenas? Did that change your mind about Trump?
That's why it happens. Their electorate only hold their "enemies" accountable. Dems do this. GOP does it (a lot).
Re: your comparison of garlands decision you linked with being concerned about our democracy if we reelect someone who tried to start a coup - can't help you there. That's a jump made by someone who will never vote for a Democrat no matter what.
Can we at least agree there are realistically some justifications to refusing a congressional subpoena? Executive privilege is one of them.