So your suggestion is that new mechanics should only be implemented if they are absolutely perfect and bug-free? (if you mean that by "finished")
Perfect/bug free? I wish but reality isn't that kind.
Functional? Yes, we should at least be reaching that standard. There are 1000+ hour vets trying to explain to me how this thing works and in the process of doing so demonstrating they don't know all the nuances. That's not functional.
I strongly disagree that fort system net-improved the game compared to alternative implementations. There was more than one way to kill carpet sieging, and not all of them were engine-breaking.
I realize that, but usually, if you beat up an AI to the point of a favorable peace deal, they are hardly a threat anymore. If you can beat an AI once, you can do it again, specially since said "again" includes you chipping off a good chunk of development away from them into you. Might be helpful to knock down a major power like Ottomans or Russia down a peg for an easier round 2, but given, round 2 isn't supposed to be too hard anyway.
It's not a huge deal, just a nice time saver. Bankrupting someone into smaller bites/less war score to completely remove them later is helpful. -3 stab might not doing, but if it procs disaster too it might and the price on this one is not exactly high.