Biden's abilities are a bit like Donald Sutherland's acting now. He does everything sitting down.
The reality is they are both too old. Biden is more doddery than Trump, that's not the slight you think it is. Its simply the reality. Trump's grasp of the facts has always been poor. He had numerous gaffes pre-2016 as well. That's why it's not a surprise to anyone.
Whereas Biden used to have a grasp of the issues. When was the last time Biden had a press conference? His team of handlers don't like to let him off the leash that speaks volumes. I'm not sure why you can't see that. I genuinely wouldn't be surprised if there are no Presidential debates this time round for that very reason.
I wouldn't vote for Trump either (I'm not a US citizen anyway) but he's more cognitive than Biden.