Firstly, I made a mistake saying 2019-2020 but we are talking about 2023-2024.
Secondly, do you hear yourself? People decline over time. In January 2023 Biden wasn't as cognitively impaired as he was in July 2024. Not only do people with dementia get worse over time, but they can often hide the true severity of their mental decline before things get truly bad.
I think in January 2023, Biden knew how bad his dementia was, but he was taking steps to hide it from those around him, out of sheer pride. Sure, there were probably signs, but enough for the DNC to step in and refuse to allow him to run for re-election?
You are talking about an unprecedented situation where a US president was, most likely, taking active steps to hide his cognitive decline, rather than simply step down early like Ronald Reagan did. There is no playbook for how to handle this situation. In retrospect, Biden probably should have been removed as president in 2019, as he was likely unfit for office at that time. Then the DNC could have held a proper primary for the 2024 US presidential election. But Biden wasn't cooperating, which made the whole situation difficult.