As one who worked at Apple stores for 9yrs 2% of users upgraded ram after purchase.
2% - that you know of. How many upgraded without ever informing Apple (oh please don't tell me Apple "phones home" and gives this "valuable research information" to Apple without informing the users, please don't tell me that).
Every Mac I've ever owned got either more RAM, a bigger drive, or both before I retired it, all without any help from Apple. Some of them even served as Linux or other no-MacOS boxes once Apple stopped allowing any supported version of macOS to run on the old hardware.
Macs are worth 75% retail value after 2 years
Yes, yes, very much yes. I do buy used. Howerver, I won't be buying any of the recent-vintage used Mac notebooks precisely because they aren't user-upgradeable.