It's all about analyzing your risks, mitigating potential damage, having contingency plans in place.
Yup, and when the AC dies in the middle of the heat wave, the stores that sell window AC units will be out of stock. Good luck with that one.
Let's look at OP's location: Finger Lakes region, NY. The average high is 83°F in July, with lows of <60°F at night. The worst they're going to experience is mildly uncomfortable afternoons that quickly turn into cool summer nights. I can endure that for a few weeks if it means not spending $6,000 years before I had to.
If OP can't tolerate a few weeks of 80 degrees, and is worried about his system failing, then he could be "proactive" and buy a window air-conditioner now for $200 as an insurance policy. That's better than $6,000 now to replace a system that could go another 10 years.
I would never tell someone to just simply replace stuff "just because". Being proactive *****- why replace a water heater before it starts leaking all over the place?
That's apples vs. oranges. A water heater that lets go on a finished level of a home can cause tens of thousands of dollars in damage. An air conditioner that let's go in an upstate New York house is just going to cause tens of days of mild discomfort.
My water heater lives in my unfinished basement with a stone floor. Our last water heater just went
27 years before it started leaking. We would have replaced it 2x unnecessarily by now if we were "proactive". Even if my heater was in a finished space that could cause damage, I would do things to mitigate risk before replacing it proactively. I'd have a large pan that drains outside with an alarm sensor and an emergency automatic shut-off valve. Failure proof? No, but neither is a new heater, either.
Spare parts can go bad. Seals in a gas valve. Circuit boards can deteriorate (although this is less common). Why tie up a couple/few hundred dollars in spare parts inventory "just because"? That's a waste of money too. An igniter and flame sensor, absolutely have an extra one. Those don't go bad in the box.
There is no guarantee a brand-new furnace will be reliable either. New devices are statistically far more likely to fail than ones that have been running a while. Look up "Bathtub Curve".
A few hundred dollars for a control board and a gas valve to put on the shelf is an insurance policy. If you pay $6,000 for a furnace and replace it every 15 years - you're effectively paying $400 year. You could replace the gas valve and circuit board every single YEAR in your existing furnace for that amount.
It's up to the OP to decide what sort of risk he's willing to deal with. For me, $11,000 is a serious chunk of change, and I'd rather just keep a gas valve on the shelf and a 10k BTU window unit in the basement if my HVAC let go at a bad time. I'd basically only do a full replacement for a failed HX that's NLA. For me, I've calculated the risks and decided it's something I can deal with it if the time comes. Maybe for OP, that's not the case.
