We''ll never know how well TOS might have done if they had gone forward into production based on "The Cage." We can only speculate.
I dunno, I think we have a pretty good idea. It's fairly clear that a huge factor in the show's popularity, possibly the single biggest factor, was Shatner's charm & charisma, and his chemistry with Nimoy. With Kelley too, but most importantly with Nimoy.
If they had gone forward into production based on "The Cage", with Jeffrey Hunter as the captain and without Shatner as the show's center, it seems likely Star Trek would have sunk without a trace.
That sounds like a slap at Hunter, and I don't mean it that way. I like him in other things, and I like his Captain Pike. But first-season Shatner brought something incredible to Kirk, and Star Trek is built on Shatner/Nimoy.
I see it this way also. Unmistakeable energy and synergy from Shatner, Nimoy and Kelley. I think beyond the great concept, actors and characters that we saw in The Cage, there is an unseen factor at play between the Big 3 that is rare, invisible and.....unmistakeable.