Canon Shmanon. The reason why to some people Star Trek – Enterprise looks and feels so much closer to The Next Generation, Deep Space Nine and Voyager than The Original Series is that chronologically it was produced closer to those shows. It's as easy as that. Storytelling conventions have changed over the decades, as have standards for production design and special effects. It is beyond me how some seem to be weirded out by the fact that Enterprise doesn't feel like a show from the 60s. Well, guess what, it isn't.
I agree 100%, however that said Relics, Trials and Tribbleations, and especially In a Mirror Darkly proved that it was possible to replicate the look and feel of the TOS era in a modern production. I'll defend Enterprise to the end, but on this point I will concede that In a Mirror Darkly in a sense shot Enterprise in the ass by proving that they could have given the show a TOS look from day one.
That said, there's a lot more to "the TOS style" than just set design. There's music, writing style, even performance style to take into account. So it probably still wouldn't have felt like a "true" TOS prequel, no matter which way you cut it. It was probably impossible to do so, which is probably why more than a few "bashers" were of the opinion the prequel simply should never have been attempted.
Alex