While I agree that quality of the execution should not be compromised - I'd rather have a great dreadnought game than a mediocre ironclad one - I would argue that the possibility of expanding the scope of the game, potentially *after launch* or even as a paid extra offers no disadvantages to the game. On the contrary, it might be beneficial financially as it could attract a wider audience and, if released as an expansion pack of sorts, would generate *additional* purchases, while still utilizing the default game mechanics (the developers would NOT be required to redo everything from scratch as a separate game).
Adding ironclads does not necessarily mean reinventing the wheel. IMHO they fit quite nicely into the existing framework - the Monitor and Merrimac are the proof of this. For example, muzzle-loading cannons don't really need to use entirely new code - in effect, simply extending the reload timer would be enough to differentiate them from breech-loaders, along with a respective research tree in the campaign. The biggest 'bottleneck' would be the amount of new hulls and ship parts for the old ships that would need to be modeled from scratch, but as the developers already stated, "some" will be available by default to cover the early campaign period. The rest is possible to achieve through "simple" tweaking of existing mechanics' values. Of course, nothing is ever "easy", but I'd definitely say it's doable.
Of course, if we get mod support in the end and custom ship parts along with it, it's quite likely we'll see some user generated ironclads in time.
On a similar note, I wish we'd get gunboats and fleet auxiliaries in the campaign too...
Screenshot is from the game 'Ironclads II: Caroline Islands War'