In the days after the debate when the dems decided Joe needed to go, there were multiple reports quoting dem donors and as well as dem staff giving first hand accounts of meetings with biden where it was evident he was impaired.
Again, just a casual observer who only saw om on TV should have been able to see it. However, if it was clear to dem donors and staff based on personal interaction, there is zero chance the media, who constantly references inside sources, was unaware.
What possible other explanation could the be other than purposefully cover it up?