According to actuarial tables it's actually significantly more valid now than it was before. There's something like a 90% chance that Biden experiences cognitive decline in the next 4 years (assuming he lives that long). And that's not surprising since 50% of people over the age of 70 experience cognitive decline in America (and it's higher for males).
Ignoring it last election for both candidates was a mistake. And Biden and Trump should both in theory put out a "I'm fine but if I develop dementia during the next turn this is the plan" plan. For Biden, that will help a lot of people who are on the fence between him and nobody see him as a better candidate and turn out in higher percentages.
Low turnout is the main thing Biden should fear. He will loose a low turnout election.
Sure but how long would he try to hold onto power? What level of decline would trigger his resignation? What things is he doing to ensure VP Harris is ready for a takeover etc...
Just showing that he has done the planning necessary is a huge step. Nobody wants another vegetable like Dementia ridden Reagan or Stroked out Woodrow Wilson; whose stroke status led to him botching the WW1 peace process and directly contributed to WW2.. Right now his (Biden's) campaign is pretending that any sort of cognitive decline is a complete impossibly and that's just irresponsible.