You are right the founders never envisaged USSC judges sitting for 40 or 50 years because the job was physically demanding.
The first SC Judges road circuit. They actually spent a lot of time on horse back riding from court house to court house. In good weather and bad.
After congress created new courts (Federal appellate courts) to perform these duties the job got easier and they could stay in Washington D.C.
“You are right the founders never envisaged USSC judges sitting for 40 or 50 years”
Because when they wrote the Constitution, people didn’t often live past 60! They didn’t account for advancements in medical science.