wrote:

You can convert a 30-year-old 60% efficient furnance to
closed-combustion with some ducting and modification to
the cabinet's venting for example.

This furnace "modification" suggestion sounds very dubious

Not really. You can create a closed-loop for the intake air very
simply with the appropriate additions of ductwork that takes outside
air directly into the cabinet and sealing the cabinet so that the
combustion side can't mix with household air. The secondary flue air
intake (directly below where the flue starts) can also be sealed and
ducted so that it pulls air from a dedicated outside supply duct and
not from household air near the furnace. So in this scenario no
outside air can enter the house through the flue, and no combustion
products or gasses can enter the interior air of the house.

That is exactly what mid and high efficiency furnaces do, and there's
no reason the same mods can't be done with low efficiency furnaces
that are 20+ years old.

Like people screwing around and winding up killing a house
full of people with CO or fire.

You're actually making it safer by creating a closed loop. By doing
so, there is no way that the blower can suck air back through the
flue. Given an un-modified low-efficiency furnace, that's exactly
what can happen if the house is sealed too tightly and too many
exhaust fans are operating in the house.

If you have a 60% 30 year old furnace and it's worth
increasing it's efficiency, then it's worth getting a
new furnace.

The furnaces made today are riddled with problems with short-lived
heat exchangers, sensors and electronic controllers.

Have a look at this site for an example of a poorly-designed secondary
heat exchanger:

http://furnaceclaims.com/

It's generally accepted within pro hvac circles that there are many
new components, ideas and manufacturing techniques cropping up in
furnaces over the past few years that don't have the benefit of years
of field longevity and durability testing.