They quit working when they’re not properly maintained. That’s all there is to it.
We were in a similar position, but just poor as heck, and a couple of times every winter I was down in the basement trying to get the heat fixed again. It was always little things. Bearing starting to fail, shaft keys getting loose and pulleys spinning, thermocouples failing. The units were 20+ years old. They were inefficient AF and really needed to go.
Now? We make sure to have the HVAC serviced yearly. It’s a couple hundred bucks. I travel a lot for work and it’s down to 20°F at night. Can’t have this stuff failing on the fam while I’m away. I can’t remember the last time it failed.
What they’re saving in maintenance they’re probably losing in efficiency. They’ll be able to carry the units only so far until one inevitably fails, irreparably, on a holiday weekend and it will cost a fortune for the weekend/emergency service rate to get someone to come by and repair or replace it. Worse yet if it fails while everyone is away and now you’ve got frozen pipes and water damage.