IMO trains should be named on the basis of rake composition and the concept only and not on the basis of train number. IR itself is least bothered about train numbering pattern and multiple exceptions already exist.
Sk concept was introduced in 2005 for providing the common people of all states, fast Delhi connectivity with less no. of stoppages outside the home state. AC pax already had Rajdhani/AC SF option available for them. So its main focus was GS and sleeper class passengers who till then were dependent up on SF trains of those routes.