Well, you're wrong. That's when you noticed, but many others were talking about it for years. It was denied.
>so you can look at brief moments of clarity and declare him "well", so many did that
Nobody dealing with Joe Biden daily thought he was well. They were intentionally lying.
I worked in an advanced Alzheimer's ward for about 4 years when I was younger. There is a look that happens in the eyes which is a sure-sign they are effectively gone - it's like a light has been turned off. (even if they have moments of lucidity, there is a clear switch that is talked about in exactly these terms if you work in these places and are close to them every day.)
Biden clearly had 'the look' back in 2021, and was making enough gaffes for people who maybe aren't as familiar with the signs of mental decline could clearly see it.
Just because you didn't, doesn't mean everyone else was wrong and saw what they wanted to see.
If you are going to argue 'well, that's just like your perspective man' you have to at least see how that same argument can be turned towards you.
You are absolutely right that it is not black and white - I fully believe that back in 2021 he had enough moments of lucidity (which generally are somewhat reliable, which appear to be tied to the circadian rhythm hence 'sundowners') -- so if all you watched were his scheduled speeches I could see how you may have been left with that impression.
There were plenty of other opportunities to watch his decline in real time however.