When my late Nana got into her eighties there was one phrase she’d often utter with a sanguine smile in response to the daily dramas the rest of us would fixate on: “Well it doesn’t worry me, I’ll be dead soon.”
Not every octogenarian is the same as your grandmother. It’s not really fair to 80-somethings to judge them based on personal anecdotes from your own family or to operate off of stereotypes about elderly people in general, such as labeling them “close to death” or “done with the world.” A common observation about people who make it well into their hundreds, for instance, is that they were quite lucid at age 80 and 90. Why should I be stuck settling for a less qualified 50-year-old candidate if a vastly superior 80-year-old is available?
I think there is no need to conflate age limits and term limits. You can, for instance, be opposed to 5th-term 70-year-old senators and be for 1st-term 80-year-old senators. There are many valid arguments to be made in favor of term limits that don’t have anything to do with age, and you have noted some of those.
I’d happily trade in the occasional Bernie Sanders if it meant getting rid of all the Dianne Feinsteins and John McCains currently coasting from term to term by demonstrating loyalty to Wall Street, big oil and the military-industrial complex. Congress shouldn’t be about individuals anyway, it should be about the American people.
If history is any indicator, you’d be trading these Feinsteins and McCains for Kaines and Cruzes. That’s not really any better. The hope is that all four of those get voted out of office because they routinely legislate against the interests of the electorate, not that two of them get ejected for being old and get replaced by younger versions of themselves.