Even setting aside one's like or dislike of Trump, the health (both mental and physical) of those who want to be President of the United States (commander of the largest nuclear arsenal in the world) is always an issue. As the Lincoln Project points out it was certainly considered fair game by the Trump campaign in 2016 against Hillary Clinton (some of the innuendo was shared here on WC2 as I recall) and they're raising it about Biden as well so they really can't complain when questions about Trump's health (that seem to have more basis than the questions he raised about Clinton's) are raised. They will complain, of course, but that's beside the point. And in the last couple of election cycles anyway the US seems intent on having senior citizens as presidential candidates - which does add to that concern.
Even recognizing the privacy concerns there should be some requirement for medical and financial records to be made public on those who want to hold this office. Going back over the years, few Americans knew that over the last couple of years of his term Woodrow Wilson's wife really governed the country because of his massive stroke, few Americans knew how sick FDR really was, few Americans knew the seriousness of Ike's heart problems, few Americans knew that JFK was essentially a drug addict because of his chronic health problems, few Americans knew that Reagan was likely at least in the early stages of dementia in his second term anyway. Trump potentially has a whole range of pyschological issues that have been talked about by mental health experts and who knows about his physical health. Biden is four years older than Trump. US voters should probably know more about the mental and physical health of both candidates.