I would agree with your position that it probably was natural causes.
The autopsy will help either confirm it or show something else.
I'm sure it will be as extensive as the autopsy of Ron Brown ... Oh, wait; they didn't autopsy Brown because it was so obvious how he died ... and then the xrays "disappeared"; the navigation chief "committed suicide", etc ...
There is no evidence that AB's death was anything but a heart attack brought on by natural causes ... but an independent source should be keeping an eye on things so all possible evidence doesn't vanish.
But it in not out of the realm of possibility that AB died from a heart attack at the same time some kook in PA sent Rush a hare-brained business opportunity - in fact, I would trust that is the case. Trust, but verify ...