Yep, I agree 100%.
Especially today. Fifty years ago, it might have been someone like Ellington or Armstrong. But, with the passing of time, Miles' importance and reach seem far greater.
Years ago, I remember reading an article in one of the jazz magazines -- probably Downbeat or Jazz Times -- about people's entry points into jazz. Miles' name came up again and again and again. It was crazy!
As for "Why is that?", I think Miles had many things going for him. Foremost, he was a musical genius -- but it was much more than just that. Miles' music continually evolved, so there are MANY stylistic doors through which listeners can enter: bebop, cool, hard-bop, post-bop, funk, rock, pop-jazz, and so on. He also had Big Red, Columbia Records, behind him, making sure that he wasn't going to be below ANYONE'S radar. Finally, his larger-than-life, protean persona also helped make him an icon. He's always had the cachet, the "X" factor. Brooks Brothers dapper or Prince of Darkness. Either way, Miles was --and is -- the epitome of cool.
I think it's all those factors that make Miles THE starting point for listeners coming to jazz for the first time.