the optics of it will look like he's beating up on grandpa.

That's where POTUS pulls the compassion card when Biden's dementia kicks in. Trump will kindly escort Biden off-stage or to a nearby chair, with a patronizing "It's OK", then treat the remainder of the debate as an interview with the moderator(s).

Take care of the downside (dealing with a mental patient), and the upside takes care of itself.