I don't really want to update it, but am going to have to, as it no longer receives security updates from Apple, as you say won't run latest XCode, which also homebrew complains about and says it can't support my OS, and increasingly software I want to install won't cause the OS is too old.
But there are zero features of an OS update i want. I am terrified of what's going to happen when I update.
However, i still far prefer MacOS UX to Windows, myself. I just a quite fine with it on 10.12.
And my 2015 Macbook is still running fine, by the way, have no need of an upgrade for more CPU or RAM or anything like that.