A new macOS version does not always make some older Macs obsolete. More commonly, a new version of the OS could make some older apps obsolete, for that new version. Likewise, a new OS that won't run on an older Mac does not make that Mac obsolete. It will still run on its current OS.
For me, the thing that makes me replace a computer has been that the content on the Internet makes the highest versions of all browsers I use obsolete. When I can't do things online that I need to do, it's time for a new Mac with a browser that is still being updated.