Our furnace is about 5 years old and I had it serviced in 2019. I wasn't planning on having somebody out to look at it but maybe I should because like you said, I can look in but hell if I know what I'm looking at.We have a brand new a/c and furnace...so yearly stuff is included in what we paid for like 5 years or something at no cost. Our old furnace was easy enough to check stuff on, these new furnaces though.. I'm not even sure I'd know what I was looking at inside. Face HE stuff that I know nothing about. I'd still have our A/C checked/cleaned yearly though no matter what.
Let's face it, unless I smell gas or the flames look off, I won't have anybody out to look at it. I have to admit, I love that burning dust smell when you turn on the furnace for the first time.