I would think a downside to living long would be managing loss...after a certain age, that is what life becomes--managing loss.
A loss of bodily functions, of friends who die, of memory, of earning power, ability to cope, ability to remain patient...it's all about managing loss...
But if I got to live a long life I would strive for the upbeat, cheerful side, seeing my children grown and thriving, parenting my grandchildren, watching my grandchildren grown and learn...my own father died at 45, so I am keenly aware that time is not always on the side of people in my family of origin...
But I shall TRY to live longer...and I shall TRY to be grateful and productive with the time I have...and to help those whom I can along the way...those things should be the price for living a long, full life.