Does the rear of the iPad act as a heatsink?

Surprisingly No!

The SoC chip is not mounted in a way to leverage the bottom case for heat dissipation. There is EMI foam which meets up with the bottom case (in your image the silver areas are raw metal unlike the gold anodized finish which are touched). The black square just under the SoC chip is to insulate the small components on the bottom side of the logic board.

What you are seeing is good engineering here within both the hardware & software so excessive heat is not created.

As to lifespan of either the SoC or the batteries use is more a factor here.