If you can't buy parts a decade after something is purchased, the repairability is a gimmick, a sales trick.
I'm not making a joke, that's the truth of it, imo.
That's how old the fairphone is.
My lgg3 is a year younger, and it's a pain in the ass to find a real battery, but LG didn't sell the thing with the idea of users being able to repair and upgrade. You expect an LG phone to have poor parts availability after a decade.
Like you said, a phone under normal use should last a decade plus. Barring failure of the main board, which is kinda where replacing that part means it's a new phone rather than a repaired phone, if you're still left with a device that you can't get parts for, it's landfill waste. Kinda puts a damper on sustainability as a factor.
Fairphone is a gimmick, and it always has been. A good gimmick to be sure, but a gimmick.