i don't think anyone could honestly argue that Biden hasn't gotten physically frailer, but TBH, I don't care about that, it's not a prerequisite to do the important parts of the job well. Shrub was healthy as a horse, he was a disastrous president. To me the only place it figures in the calculation is wrt whether he lives out his second term, but even on the grounds that Harris is a largely unwritten story, i'd still vote for her even up over Trump or anyone Trump is likely to picks as a VP so even there, it doesn't weight negatively on the scale.
When I look at the operation of the Biden administration, I don't see any lack of vision, consistency. I don't see idiosyncratic actiions, senseless windmill tilts, or other evidence of an erratic hand at the tiller - so that's what I have to go by.