I would say the following:
* I also perceive that Biden has definitely lost a step or two in the last 4-5 years.
* Even Sanders' campaign has made an issue of this. They usually did it by comparing energy levels between the two. But it was pretty obvious they were alluding to a claim that Biden could not stand the rigors of a full on campaign or being President.
* It was predictable that the Trump machine would characterize it as full on dementia or Alzheimer's, though that is quite an exaggeration, in the extreme. There had long been hints of it. But I saw Giuliani on Fox the other night and he was outrageous in his claims, all with a smirk on his face, that Biden has some diagnosable mental loss or infirmity (I pause to note the irony of that coming from Giuliani).
* Time will tell. I do believe he will get the nomination and we will see how he does on the trail for the next 6 months. Based on what I've seen it is not nearly as extreme as you make it out to be, but if in fact there is a real problem we will certainly see it through that process.
* Trump needs to be careful about how he addresses this. I really do not see in Biden any effect beyond normal aging. If Trump and his team go overboard on how they characterize that, they risk alienating the entire population in that age range because they will identify with the minor things observed to date.