In a 10 man, 5 mages bring your more damage over shamans than 5 stormlashes.
In a 25man there may be a balance, but in the end stormlash is not mandatory because the raidwide damage could be chalked up to the fact neither spec is top-of-the-pack material. A 1% (and possible sub 1% when not on fights like elegon) raidwide damage increase is much weaker than many things.
Your argument of stormlash effectively means it would've been impossible to raid without the 3% raid damage buff in cata. Plenty of people did it. It was nice to have but not the end of the world if you didn't.