I don't recall anybody suggesting dismantling the fleet.
Except for when they literally used those exact words or said something similar multiple times (see below).
That they may have renegotiated terms with the Klingons later to preserve the defensive arm of the fleet, or whether they simply beefed up the defensive capabilities of their exploratory and scientific ships is irrelevant to the fact that Cartwright and other high ranking officers were apparently (over)reacting based on the possibly erroneous assumption that the defensive arm of the fleet was going to be eliminated as a result of the treaty.
It wouldn't be the first time high ranking military officers, government officials, and politicians made rash decisions based on hyperbolic and reactionary preliminary readings of a chaotic political/military situation.
SPOCK: Good morning. Two months ago a Federation starship monitored an explosion on the Klingon moon Praxis. We believe it was caused by over-mining and insufficient safety precautions. The moon's decimation means a deadly pollution of their ozone. They will have depleted their supply of oxygen in approximately fifty Earth years. Due to their enormous military budget, the Klingon economy does not have the resources to combat this catastrophe. Last month, at the behest of the Vulcan Ambassador I opened a dialogue with Gorkon, Chancellor of the Klingon High Council. He proposes to commence negotiations at once.
CARTWRIGHT: Negotiations for what?
SPOCK: The dismantling of our space stations and starbases along the Neutral Zone, an end to almost seventy years of unremitting hostility with the Klingons, which the Klingons can no longer afford.
MILITARY AIDE: Bill, are we talking about mothballing the Starfleet?
C in C: I'm sure that our exploration and scientific programs would be unaffected, Captain, but...
CARTWRIGHT: I must protest. To offer the Klingons a safe haven within Federation space is suicide. Klingons would become the alien trash of the galaxy. And if we dismantle the fleet, we'd be defenceless before an aggressive species with a foothold on our territory. The opportunity here is to bring them to their knees. Then we'll be in a far better position to dictate terms.
KIRK: Sir!
C in C: Captain Kirk?
KIRK: The Klingons have never been trustworthy. I'm forced to agree with Admiral Cartwright. This is a terrifying idea.
SPOCK: It is imperative that we act now to support the Gorkon initiative, lest more conservative elements persuade his Empire that it is better to attempt a military solution and die fighting.
C in C: You, Captain Kirk, you are to be our first olive branch.
CHANG: Tell me, Captain Kirk, would you be willing to give up Starfleet?
SPOCK: I believe the Captain feels that Starfleet's mission has always been one of peace.
CHANG: Ah.
KIRK: Far be it for me to dispute my first officer. Starfleet has always been...
CHANG: Come now, Captain, there's no need to mince words. In space, all warriors are cold warriors.
http://www.chakoteya.net/movies/movie6.html
“And if we dismantle the fleet, we’d be defenseless before an aggressive species with a foothold on our territory.”
That statement in itself was a valid point.
What I found to be odd was the CinC’s response to the aide’s question, “are we talking about mothballing the Starfleet?”
He could have answered with a “no”. That answer would have been concise and to the point, if there was absolutely no possibility that was going to happen.
Why did the CinC fiddle around with his answer about “exploration and scientific programs would be unaffected”? Maybe the reason that he responded the way that he did was because there were indeed proposals on the table (i.e. negotiation table) for a substantial dismantling of Starfleet.
It seemed more likely that had he continued his answer, it might have went something like “I’m sure that our exploration and scientific programs would be unaffected, Captain, but”
1.) the military arm of the fleet would be cut severely, if we agree to the terms proposed.
or
2.) I am being deliberately vague so that it will prompt some future (past - 21st century) internet forum to have a heated discussion about what we all said here in this scene.
During the dinner on board the Enterprise, Chang asked Kirk, “tell me, Captain Kirk, would you be willing to give up Starfleet?” That conversation at the dinner led me to believe that the dismantling of Starfleet was probably one of the terms (at least initially) that the Klingons demanded of the Feds.
Back to the scene in the Starfleet meeting, after Cartwright asked “negotiations for what?” Spock responded, “The dismantling of our space stations and starbases along the Neutral Zone…”. That might have been the Fed’s counterproposal. But even Spock’s proposal called for some degree of dismantling of Starfleet.
Both sides apparently were intending to negotiate the dismantling of some or all of Starfleet. In that context, Cartwright objections seemed valid. Ultimately, what was actually agreed to was the Khitomer accords. But at that point in the movie, nobody knew exactly how much Starfleet was going to be dismantled or reduced through the negotiations.
This is not to suggest that because Cartwright made some valid points that he was a good guy. He may have been right but for the wrong reasons. He may have had darker motives. He turned out to be a treasonous conspirator.
