Its not just about aircraft handling. Its also about being in command.
Being an instructor means you have to make 'command decisions' early in your career...and expose yourself to jeopardy. Someone who has gone through a CPL/IR (especially if on an integrated course) will not make their first genuine command decision until in the left hand seat of an airliner.
As an instructor you make flight safety decisions on a daily basis...and there is often no person to turn to to see if that decision was right/sensible. Decision making skills get honed (with immediate feedback if your decision making is poor). Just the kind of thing to have when you're faced with a problem that the SOPs cannot guide on.
I have 1000 hours pilot in command and have yet to get an airline interview (let alone a job). These hours have been 'in command'...i.e. if it turns to sh1t in any way then it is my role to get it back.
As the axiom goes. A superior pilot does not need to use his superior handling skills, as his superior judgement and decision making skills help him to avoid the bad situation in the first place.
Flying Instruction allows an instructor to make the mistakes (at slow speeds) that allows an instructor to develop the judgement to stop it happening again when it may be for real..in something faster and heavier. I acknowledge that FIs may pick up some bad habits and (lets be honest) arrogance...but an apprenticeship as an FI has worked for the last 80 years of aviation. Why the sudden change?