David Radlin wrote:
I would expect the life
of one of these things would push 40 plus years with the occasional
component replacement. I'm thinking that lots of things would have
to fail in concert for replacement to ever be cheaper than fixing (or
many things pending failure).

What components commonly fail in these things and what are the major
component design lives?

According to Appliance magazine the average life of a gas furnace is 20
years. YMMV.

In my experience the failure modes are the flame sensor, the ignitor,
the gas valve, limit switches, the inducer motor and the heat
exchanger...not necessarily in that order. On furnaces with electronic
controls the control modules are subject to failure.