The point does stand generally though. I have had to replace entire HVACs because the coolant is no longer available or because the cost to replace a certain component is too high because the parts are no longer readily available.

I think the only solution is to try to build as minimally as possible. A huge amount can be accomplished through passive heating and cooling (windows placement and size, site location and angle toward prevailing winds and morning / afternoon sun, etc etc). We are too often reliant on technology to allow us to ignore these things and we do so at our peril.