Mea culpa, displaying my isolative ignorance of the reality of the times here!!! My thought was to point to the less pervasive, encompassing scope of online generated awareness, to the society at large, of popular media buzz 8 or 9 years ago. To the slight degree that that perception even has much credence is irrelevant anyway, as publicity concerning anything relating to Enterprise wasn't likely to make much of an impression on more than the very outer periphery of the prevailing zeitgeist. Now taking much time and effort in removing foot from mouth!!!!
Short answer: "But it's ... it's Bill Shatner!" (paraphrasing Dax) Long answer: Shatner probably would overshadow the rest of the cast, but the potential rewards would be huge if the audience perceived the interaction between Shatner and the crew was genuine. Of the five times a TOS regular appeared on one of the spinoffs, only one agruably fell flat: Flashback, the one that was most structurally alike TATV. It's difficult to overestimate Trek's debt to Jonathan Frakes (or his sideburns), but his star power was as big as other actors (especially with the audience outside fandom). Spiner, who has a much higher profile than Frakes, drove the episodes he appeared in, but arguably didn't outshine the cast the way Frakes actually did. (And as I argue repeatedly, a lot of that simple reflects the narrative structure: the gimmick that Braga used to bring the characters together.) Having Shatner (or Nimoy or even Stewart) could have created a sense of a special connection between ENT's characters and and a beloved hero. And if anything, the finale should created the sense that the NX-01's crew contributed to a great saga, Valentine or not.
Shatner would have been too old and fat to do Kirk. And I say this as someone who adores Shatner. Voice over, sure. Frakes seems to be a nice man but Riker is a crud bucket. I would have forgiven any Shat, ANY SHAT AT ALL. Because, Kirk. But poor Frakes had to portray a sleazy turd and so ruined TATV, though other things also ruined it like you know the whole concept.