Even counting in the furnace under $1k if it’s low efficiency, or $2k if it’s high efficiency is not that bad to replace. Gas furnaces are very simple units, chances are if anything goes it will be a control board or heat exchanger, both plug and play parts. Furnaces should go for 20+ years. The ac I can see how it’s a bit of a time bomb , they have next to no serviceable parts, and are delicit by design. Check out the replacement cost of the unit.
Even at 8 years, it’s hard to imagine everything going over a short period.
If you are at all handy, can follow directions, read, you should be able to diagnose your problems pretty easily, and make an educated rescission on when to repair or replace. If your the type of person who would call a repair guy when something went wrong and be at their mercy, maybe I would consider the insurance as a lesser of two evils.
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