It's funny. The defining of Kirk's Enterprise as "Constitution Class", didn't become canon until 1987. In, "The Naked Now". IIRC.
Sure. Next year it may turn out that a Daedalus has a spherical hull or that it looks like Franklin from Beyond. Or something entirely different. A lot of fans were upset that the "disastrous first contact" between humans and Klingons didn't turn out to be an overtly violent incident that immediately led to all-out war. They were convinced that Picard's remark in "First Contact" established this unambiguously. Trouble is, that was never the intent of Picard's line to begin with. It is now canonical that twenty years before Kirk commanded Enterprise there were ships with saucers several times the size of the main hull of the TOS Enterprise. In 2009, fans were not expecting that.
Yet even there, we know of many Federation starships that have shared a repetitive name but not the same design. You could show me a ship with a saucer called "Essex" or a ship with a globe called "Essex" and unless it were said that this Essex was a Daedalus, there would still be no canonical Daedalus design.
If I understand this reasoning correctly, there is also no such thing as a Miranda-class starship and I don't think that set of words was ever used in canon or attached to any ship.
Perhaps not. Not that there's any such thing as any of these ships, which is why anything can change.
The U.S.S. Brattain (using the Reliant studio model from TWOK) had a dedication plaque stating that the ship was Miranda class. No, you couldn't see the info on the plaque in the episode, no one spoke the class name out loud, but it was still there nonetheless. The Reliant-type ships had no class name until the particular TNG episode that the Brattain was in.
The spherical hull model from Sisko's office had a ship name and registry. No, you couldn't see the info in the episodes, no one spoke the ship's name out loud, but it was still there nonetheless. The same rules should apply every time.
but what about teh haed cannon my head is pretty much made of cement, that's basically stone isn't it
No one is saying anything different. IIRC, the ship in Sisko's office is labeled "Horizon". So there is still no connection between the model and the Daedalus class.
Except that this is one theory that the company has remained consistent on in all secondary sources and tie-ins.
It's not canon, but it certainly suggests they would stick to it. They don't have to, but they have the option. Like I said, non-canon things have become canon before, such as Sulu and Uhura's first names, Kirk's middle name, Shi'Kahr, Vulcan's Forge, sehlats, Caitians, IKS Klothos, Antares Type ships, and no doubt many others. Non-canon ideas are, by definition, non-canon, but can, should, and often are used to enrich the canon Star Trek universe.
No one was arguing about the desktop model's name and registry. The argument was whether it represented a Daedalus class starship, which the jury is still out on, because there is no canon source linking the two things together, other than Okuda's conjecture. Okuda's reference work has the two as one and the same. The reference work is not canon. Granted, the info in the reference work has snowballed to other reference works, but the fact remains that nothing on screen has so far proven that the two items are the same.
Kirk's middle name? We all know it's James R. Kirk. As for sehlasts and Caitains, I don't know how much that was considered "non-canon" given the TAS creation. GR only dismissed it as canon later on, so I always found that kind of odd. But, I grew up reading Bjo Trimble's Star Trek Concordance, long before the idea of "canon" was given weight. As to your final point, I don't mind non-canon things becoming canon, but enriching the Trek universe strictly by those things is self-limiting. I would rather the production team feel some measure of freedom to design a ship that suits their needs, while looking at what ha come before. It's a delicate balance, but I would rather see them try something new.
According to some/many people (not me BTW) TAS isn't canon. Some of my fondest memories of Trek involve reading FASA, Federation Reference, Mastercom Data Center, Starstation Aurora and other "fanon" reference books.
Because they keep copying it. It's not like there's any independent thought or research. It just laziness. It's in the Encylopedia so MA copies the idea. Then startrek.com does the same.