No, this is not right (unfortunately). And having upgraded does not make the new OS your "native" OS. Having to return the computer to its original OS was started by Apple when they switched to the download only method. And it is spelled out in the licensing agreement. The problem is that whatever you download from the app store is tied to the Apple ID used to get it. It is also specifically not transferable. Unless you follow the steps to erase and reinstall the original OS (which is the only method to disassociate your Apple ID with the computer), the recipient will never be able to update any apps or reinstall the OS. Just to prove the point, here is a link to the licensing agreement:
https://www.apple.com/legal/sla/docs/macOS1013.pdf
(See section 3 B - here is an excerpt):
B. If you obtained your license to the Apple Software from the Mac App Store or through an automatic
download, it is not transferable. If you sell your Apple-branded hardware to a third party, you must
remove the Apple Software from the Apple-branded hardware before doing so, and you may restore your
system to the version of the Apple operating system software that originally came with your Apple
hardware (the āOriginal Apple OSā) and permanently transfer the Original Apple OS together with your
Apple hardware, provided that: ā¹ the transfer must include all of the Original Apple OS, including all its
component parts and its license; (ii) you do not retain any copies of the Original Apple OS, full or partial,
including copies stored on a computer or other storage device; and (iii) the party receiving the Original
Apple OS reads and agrees to accept the terms and conditions of the Original Apple OS license
It's not really a big hassle, especially now that you know how to do these things. So, when you're ready to sell, simply boot into internet recovery, wipe and reinstall.
And thanks for the compliment....