Look, to put this simply: A series is canon when its original elements are used afterwards and reference back to the place of origin.
TAS elements have re-appeared in Trek, throughout all of them. Especially ENT, which would've re-introduced the Kzinti.
Now, if there were an episode of the Kzinti, there wouldn't have been any questions regarding TAS's canonicity now, would there?
Let's make this into a logical proposition, shall we?
The Animated Adventures of Star Trek is part of the
Star Trek canon because it is referenced in live-action
Star Trek productions (e.g.
sehlat, Shi'kahr, Tiberius, Pallas 14, etc., etc.).
1. Principle: If a work is referenced in any live-action
Star Trek production or other canon work, then that work is part of the
Star Trek canon.
2. Fact:
The Animated Adventures of Star Trek has been referenced in live-action
Star Trek (examples: see above)
3. Conclusion: Therefore,
The Animated Adventures of Star Trek is part of the
Star Trek canon.
Interesting. Very interesting.
1. Principle: If a work is referenced in any live-action
Star Trek production or other canon work, then that work is part of the
Star Trek canon.
2. Fact:
The Andorians: Among The Clans (Last Unicorn; 1998) has been referenced in live-action
Star Trek or other canon work (example:
ushaan in "United" [ENT])
3. Conclusion: Therefore,
The Andorians: Among The Clans is part of the
Star Trek canon.
Okay. Well, that's kinda psychidelic, but
Among the Clans doesn't do a whole lot of damage on its own. It screws up the Star Charts, introduces a very interesting Andorian history involving a mass historical annihilation around the 20th Century, but that's nothing the continuity can't handle.
1. Principle: If a work is referenced in any live-action
Star Trek production or other canon work, then that work is part of the
Star Trek canon.
2. Fact: The Pocket line of
Star Trek novels has been referenced in live-action
Star Trek or other canon work (example: "The Forge" being a giant love song to
Vulcan's Forge)
3. Conclusion: The Pocket line of
Star Trek novels is part of the
Star Trek canon.
And, with that, the
Star Trek universe changes from a highly internally consistent universe unlike any other legendarium in size, scope, and continuity since Tolkien into a whole sort of general mish-mash of insanity and contradiction. You just made
The Entropy Effect canon, which plays all hell with TOS, and don't forget
Dark Mirror, which blows DS9 MU episodes to tiny bits. But wait! There's more!
Let's say, hypothetically, that that Enterprise episode
had been made in Season 5, featuring the Kzinti!
1. Principle: If a work is referenced in any live-action
Star Trek production or other canon work, then that work is part of the
Star Trek canon.
2. Fact: Larry Niven's
Known Space series has been referenced in live-action
Star Trek or other canon work (example: hypothetical ENT episode)
3. Conclusion: Larry Niven's
Known Space series is part of the
Star Trek canon.
Boom. Your principle just destroyed the universe. You're like Q, only with madder skillz.
But wait! We didn't even
need that hypothetical Season 5 episode to make
Known Space canon! You already made TAS canon, and TAS "The Slaver Weapon" is a big homage to
Known Space, therefore
Known Space is canon!
Hey, I'm really getting rolling with this!
1. Principle: If a work is referenced in any live-action
Star Trek production or other canon work, then that work is part of the
Star Trek canon.
2. Fact:
The Hitchhiker's Guide To The Galaxy has been referenced in live-action
Star Trek or other canon work (Milliways and Sirius Cybernetics were both on DS9's Promenade listing, and Tricia McMillian's name appeared on a document in "The Schizoid Man.")
3. Conclusion:
The Hitchhiker's Guide To The Galaxy is canon!
Earth is destroyed! The Vogons roam the universe! Bistromathics is a viable form of transport! Dolphins are smarter than humans! The answer to everything is forty-two!
This is all canon now!
Clearly, this was not how you intended the principle you proposed to work. It is preposturous and insane. But it
is how your idea works, logically, whether you like it or not.
References are just references. They're homages. Tips-of-the-hat, if you will. They are
not a declaration of authority for anything. So canon, whatever else we may say about it, is definitively
not determined by reference.
In a related observation, I think that the increasing tendency of Internet fans to declare new canon based on arbitrary rules independent of a higher studio authority ("I like it, therefore its canon." Yes, these fans
always misuse "it's.") is a far, far greater threat to Star Trek than any JJ Abrams so-called "reboot." We stand the risk of opening the doors to
everything, and, when that happens, Star Trek won't even be accessible to the
fans anymore. It really would either kill the universe or, more likely, force a JMS/BSG-style canon wipe. And I don't think anyone arguing about canon wants that.