I beg to differ.
the meta is stimulated BY off-meta picks, by pigeonholing champions you stifle diveristy and the meta stagnates.
A stagnant meta creates creep (power, mobility) and promotes playing FotM.
Playing FotM leads to a very small effective champion pool, and the same 15 champions every game.
A prime example of pigeonholing being a bad thing is
; a champion made specifically to be incapable of anything other than support by intentionally having no waveclear and a zoning ultimate that doesnt do much damage.
You could make braum capable of being a solo laner, and a fantastic one at that, if you let his Q pass through minions. Its a small change, and it wouldnt make braum OP OP, but it would ENABLE diversity.
The OPTION to play outside the intended role and do the unexpected is what makes MOBAS interesting to watch and play; Who would expect lulu to actually be a fantastic jungler? Galio an ADC? Annie a jungler? Udyr a support?
You wouldnt. And that surprise option is what can make or break games.