The only way to update is through the AppStore. Big Brother Steve is watching you.
Not the only way. Once you upgrade one machine you can upgrade the rest via optical/flash easily enough. You also only need to buy only ones for all machines associated with your Apple ID.
I don't really understand the whole "Apple will make OS X like iOS and stop you from "sideloading" apps" rhetoric. iOS is an "embedded" OS used on specific types of devices where it makes perfect sense to have limited functionality/ability to install since it's all about experience and making sure that things run the Apple way.
OS X is a desktop OS that is meant to be more robust and allow a certain amount of freedom to the user (basically if it can compile on it you can use it). Could Apple lock it down like iOS? Sure they could. Will they do it? I kinda doubt it, there is no reason whatsoever to do so.
1) They make a shitload of money from Mac, Apple has a fairly large markup on their "desktop (laptops included)" hardware so they make a decent amount of money out of it.
2) They make no money from free applications - a HUGE number of Mac software is free, either FLOSS or shareware or even nagware in some cases but you can very easily have everything you would need for everyday computing for free (without piracy). So forcing users to go through App Store won't really give them any more money, in fact it would make it easier for them to find free alternatives to paid software.
3) App Store doesn't allow certain things that would hugely decrease Apple's userbase - you cannot have torrent clients from an App Store, so anyone who does any kind of torrenting, which is a huge amount of people as we all know, will move to a different OS.
4) There is no real lock-in for OS X as opposed to iOS. Apple can get away with what they do in iOS because of the lock-in effect. They have majority share in the tablet space and a significant share in the Smartphone space. There are many very obvious differentiating features between iOS and Android and those who prefer one very rarely switch to the other. Also for the most part iOS and Android don't share apps, there are a few examples like Angry Birds but, at least from my experience, a majority is different and its not necessarily easy to find something similar. There is also the fact that most iOS apps are paid and if you have a certain amount of money already invested in the platform it makes very little sense to move to a different platform. OS X vs Windows is not quite as obvious, for one Windows machines tend to be cheaper, for two as I mentioned before there are tons of free applications for both and many are cross platform. (As an example the only OS X exclusive application that I use that I can't find a good alternative for in Windows is iStumbler).
5) Increased availability of web based applications - you can already run an entire capable office suit from the web, requiring only a browser. There are services that allow you to play games that are hosted on a central server (though thats not from a browser yet), online streaming solutions are ubiquitos. There is basically no way to lock a desktop down at this point as for the most part all you need is a browser and a broadband connection to use just about anything you want.
6) Games - the biggest lock-in that Apple currently has in iOS is games. More importantly very long running games like Cut the Rope, or Angry Birds. Because data is saved locally (and soon on Apple servers) switching OS's will also cause you to lose your progress. In the PC gaming this is barely the case anymore. The most popular games are MMORPGs that by very nature keep all your progress on a central server. Then you have things like Battle.net and Steam (not sure if Steam keeps progress saves though but I'm sure if they don't they will soon) that will allow you to buy once, download many with your games and lets face it there are more Windows games than there are OS X.
Bottom line is that in the desktop world OS X is rather unimportant and is used by a fairly small number of people. If Apple does attempt to lock it down the users currently on it can very easily move to Windows (even on the same hardware) as there is virtually no software that is OS X exclusive and has no alternative in the Windows world. So for Apple to lock it down would make no sense whatsoever and at best have very little effect on their bottom line, at worst deplete OS X user base to where they were in the 90s, so basically only people who do any kind of graphic design.