While there are many reasons behind this (including Mac OS upgrades typically are not complete overhauls in the way Windows use to be), the major underlying factor is Apple saw the advantage of assuring all users run the same OS. Microsoft continued to follow this path until Apple bucked the trend and started to offer its operating system upgrades for free.
Microsoft, for example, would charge several hundred dollars (depending on the version) to upgrade. Such a big deal in fact that companies would charge hundreds of dollars for the privilege to upgrade. Operating system upgrades use to be a big deal. This makes upgrading to MacOS the right call. Performance likely won’t drop off by not installing MacOS Sierra initially, but as subsequent patches come out a non-upgraded system may grow sluggish. While not a vital upgrade, it does ensure programs (especially Apple software) runs smoothly. Apple regularly releases new operating system updates to users for free.
