SAP basis is also SAP system administration like Linux admin or window admin. So you would like to possess some OS admin understanding.
But I believe SAP basis consultants are abundant in demand and also pays well compared to OS admins.
As an SAP basis consultant, In addition to OS skills, you will also be knowing about SAP software, packages, add-on, updates upgrades, etc. And also basis configurations need to be made.
I think Basis is an evergreen branch that will exist till SAP is there. as a basis consultant, you will be the first one to touch an SAP system.