Especially when some software vendors drop support of previous versions of macOS. Recently Sketch axed support for macOS Mojave, which was released less than three years ago!
And it is a real problem because a lot of fine functioning computers can't be officially upgraded to newer versions of macOS, rendering them essentially useless.
Because all plugins, all samples and stencils on popular websites rarely support anything other than the later version. You can do without upgrading version of Sketch for maybe 6months, but then you'll run into problems, and in 18 months it'll be just what it is, cloud features gone, modern plugin versions gone too, no official libraries are of any use.
The last High Sierra update was 7 months ago and it's now unsupported. It's not going to get anymore now that Big Sur has launched as Apple only supports 2 versions back.
So no security updates for starters, software slowly starts to leave behind old versions too.
And that time is two years after the next version comes out. That's how it's worked since Mountain Lion and High Sierra's time is up which is why it hasn't gotten any for 7 months while Mojave has received some.
And it is a real problem because a lot of fine functioning computers can't be officially upgraded to newer versions of macOS, rendering them essentially useless.