I think the “party line” response to this would be that the warranties you get include preventative maintenance as well, making it a different proposition - you’re not just buying a warranty, but a full service plan after all. But I’ve never been convinced by that argument.

I just went through buying a furnace and had a few prospective installers. The first tried to sell the extended coverage: twice annual “checks and maintenance” and a 30% discount on all parts.

The second said he’d let me source the parts myself if I so desired and if I would be responsible for changing the air filter on schedule and hosing down the outside fan every summer I’d be better off putting the annual fee into a sinking fund. Or, I could pay him $200 a year to hold a hose, money he’d happily take. He got the job. ;-)