It’s not the Dems’ fault who the Republicans run, but if they end up losing to someone as widely criticized as Trump in November, it will be partly due to their decision to field a 'sympathetic, well-meaning, elderly man with a poor memory' and someone that a significant majority of the country deems unfit for a second term.
I don't feel like this is that hard to understand. For genuine change to occur, we need to recognize that both parties contribute to the existing situation, even if acknowledging this fact might be uncomfortable for you.