I've always felt that somewhere near the tail end of VR and the reunion of STP is when things shifted for the worse and never recovered. I think Scott was in a bad place in the late VR days and then he jumped straight into the huge STP reunion tour. I remember the first time I saw the mugshot shortly before the main reunion tour was to begin and remember being a little shocked at the transformation in his appearance in the short period of time since the Houdini show.
My view is that his voice, as well as himself, needed a break after the VR breakup tour, and that never happened. I think it's very possible we could've had an entirely different reunion if he was able to take a breather, recover, and get the help he needed at that time.