That's true, but your GP is suggesting that infrastructure investments are unproductive as well.

> Medicare and social security

These two are often lumped together, though are quite different. SS deficit can be fixed with slight increase of retirement age and slight payout reduction and slight contribution increase. Medical spend is completely broken and unfixable without going after the medical cartels.

The fact that it can be fixed doesn't matter if its currently not fixed. Those are the two biggest budget items, so it's what the government is spending the most money on.

Technology developed that goes into keeping 83 year olds alive actually does have second order effects on productivity and being able to work for longer.

The 83 year olds aren't having dollar bills ground up and pumped into their bloodstream. That money is being paid to doctors, hospitals, medical manufacturers, etc. All of these entities are then going and spending those dollars on the inputs they need. Increasing hospital capacity is as legitimate a thing to spend money on as reducing traffic congestion.

More to the point though, if you spend $20k keeping grandma alive, is she any less alive because that $20k was loaned versus pulled out of her mattress?

GDP is not a measure of how optimally money is being spent, it's just a measure of how much economic activity is going on.