First of all, there is a world of difference between “a bit forgetful sometimes” and “mentally unsound”. It’s possible to be effective in a leadership role while being a bit fuzzy on the details.
Second of all, I think it’s well established that Presidents deal with highly classified info all the time. Given that they basically work 24/7 and all of their residential areas are also workplaces, it is virtually impossible to keep them as separate as other citizens and military personnel with clearance do. So we should not be surprised that when they are forced to move out of their government housing in a hurry, some stuff gets taken that shouldn’t.
The fundamental question is: once we get an accounting of what is missing, what happens then? Biden (and all previous Presidents and VPs, for that matter, except for one) immediately cooperated with authorities, with the intent of finding anything that was inadvertently taken and returning it. Trump stalled and whined, and came up with excuses instead of acknowledging that he might have done anything wrong, even to the point that the government needed to execute search warrants to recover it.
And this investigation, from a supposedly independant and fair investigator, had a clear agenda attached to it. Once the underlying facts get exposed, it’s clear that this was meant to be more “buttery males”. It may be that Merrick Garland’s biggest contribution was to make sure this didn’t get released in October.