No upgrade is "necessary" unless certain software you require no longer supports whatever you're using.
High Sierra will continue to work fine for the next three or four years at least. Eventually, upgrading it will be prudent, as newly-discovered security issues will probably no longer be fixed maybe five years from now.
Comments
High Sierra will continue to work fine for the next three or four years at least. Eventually, upgrading it will be prudent, as newly-discovered security issues will probably no longer be fixed maybe five years from now.