If they implement it gradually in stages then I'm sure they could do it. The tablet is going to be App Store only for definite. After that they release a Netbook style machine somewhere between the Tablet and the Macbook. Same deal, closed App Store. It's acceptable and one step closer. Slowly more consumer orientated Laptops are phased out to be replaced by new models that too, follow the closed App store strategy.
Before you realise because it's happening over years rather than big bang the entire platform is closed off. Customers will remain, some will leave but Apple won't care as the returns are too great for them to ignore.
A closed Mac App Store is a no brainer for apple and it WILL happen.
I do agree that this is how Apple would most likely implement a lockdown of OSX if they wanted to go in that direction.
I strongly disagree that Apple thinks this would be in their best interest. The loss in hardware sales from disgruntled users, the decrease in available 3rd party software from disgruntled developers and the all-around bad press/vibe this would generate in the computing world would be an unmitigated disaster. The stock price would plummet.
Apple may not always make the best decisions, but it's not run by morons. The vast majority of its profit is generated through hardware sales, and even a 30% cut of all 3rd party software would pale in comparison to the losses from the subsequent reductions in hardware sales (especially when you factor in the costs of running the approval department and the other overhead).
Also, consider what Adobe, Autodesk, Microsoft etc. would do if OSX went completely closed. I don't see them accepting a 30% bite into their revenues, so they would either have to jack up prices to maintain profits, or (more likely) pack-up and leave. Assuming they did stick around and raised prices to compensate, Macs would gain the reputation of having an "Apple Tax" where all 3rd-party software was ~30% more expensive than the same products on other platforms. This would certainly hurt the overall market-share, and ultimately Apple's bottom line. I'd like to think that Steve, the board and the other higher-ups are smart enough to have considered this.