For one thing you have to look at the ramifications art wise the original image 7 guys brought to comics in the 90's and the seeds they laid down that you can see in todays comics.
McFarlane changed how Spider-Man is drawn, so if Todd hadnt arrived as an artist on Spidey in the 90's, what would Spidey look like now?
Notice that out of all of the main Image guys Todds style is the one least copied.
Greg Capullo's current style is derived from Todd because he wanted to keep the look of Spawn consistent with what Todd had done.
If Todd showed his work to editors he'd be hired on the spot, no doubt in my mind.
Is Todd a Legend? Well, Todd isnt dead yet so he still has time to build one. Right now I say he has left a legacy.