I don't think it's the immune system itself that is the big problem, The information that we have so far suggests that mitochondrial impact is more significant when it comes to us long haulers. (The immune system is important but doesn't always recognize threats until it is too late and/or can also overreact.) Mitochondrial impact might not be as immediately evident as the symptoms created by the body's immune response.

Covid-19 (which is the name of the initial SARS variant) absolutely wrecked and/or destroyed a notable account of lives and significantly affected a good number of others. (I'm one of them.)

The majority of us who have the worse long haul symptoms appear to have been exposed prior to a vaccination being available. This stands to reason that one of the reasons many of us long haulers catch it even after vaccinated is because our Mitochondria is already impacted and I theorize that it is sort of like having the door kicked in...

For some viruses, the door isn't destroyed and is bolstered in time...so it takes a few hits but is made stronger. Or you have ones like the Measles that essentially removes the entire door AND makes the new doors out of cheaper material...

For Covid 19, its more like that for some of us, the door is still there but was pretty battered and had cracks in it...we can ADD defenses and repair the door after the fact but it won't be as good as that original door was before it got kicked in.

And when it comes to immune response, that repaired door is good at keeping most crap out, but crap that makes its way through anyways heads right for the Mitochondria and continues to screw with us.