Except that new releases of Firefox and other software will still run on older versions of MacOS, but when Safari fails to keep up with new types of web content, you can't update to a newer version of Safari without also updating to a new version of MacOS.
Most major application developers (at least the ones that I deal with) are pretty good at making sure their new app versions will still work on older versions of MacOS. I don't see Apple as being very concerned about this. The latest version of Firefox still supports OSX 10.9, and it seems to me that the previous version supported much older versions of MacOS than that. I remember checking the minimum system requirements on a previous Firefox version, and being very surprised by that.
I wouldn't have a problem with Apple's constant pressure to update, if they put more emphasis on bug fixes and reliability, rather than on memory gobbling features that I have no use for.
