There's already a Enterprise class. And it's a Aircraft Carrier.
Erm, that was basically my point; since we know the television ship was named after the real-life aircraft carrier, perhaps the ship being the only member of her class was also lifted from real-life?
You know... who's to say, as I think someone else mentioned, that she's not the only member of a Starfleet Enterprise class, not unlike CVN-65 in real life? This explains both the plaque (and the Defiant one) and the simulator sign in TWOK. All along, from TOS until her destruction in TSFS, she was the sole member of the Enterprise class. Then, the A was a member of the Constitution refit class. Further, the Enterprise class might make use of Constitution class phaser banks.
Moving on, I don't for a moment believe that the production team had everything planned out on TOS, nor do I believe that all of the members
of the team agreed on everything. I do
think certain things were
planned, and certain things were left intentionally vague. And, some things that were planned were changed as the show rolled on; consider the various terms used for the agency to which the Enterprise
reported: Space Central, the Star Service, UESPA, and the UFP wasn't referred to for some time.
I think that's the problem - it's very difficult to sift through what was planned or not. Certain evidence taken only from TOS will give you one notion, while if you take TOS with the rest of the franchise things might appear another way. So it's understandable that different perspectives would draw different conclusions.