On the question of "upgrading hardware every year", every computer in my house is ML compatible, the oldest of which is almost five years old. So, it seems that as long as you've upgraded your hardware within the last few years, you'd be fine with this. This does make me curious about what the most recent machine that was Lion compatible but not ML compatible was...
The worst case for Mountain Lion would be if someone had bought a mid 2007 Mac Mini in March 2009, just before the new models were released. That was three years and four months before Mountain Lion was introduced.
The oldest model supported by Mountain Lion is the Mid 2007 MacBook Pro, introduced in June 2007, which was five years and one month before Mountain Lion.
Putting it another way, Mountain Lion supported all Macs introduced up to three years and four months earlier, and some Macs introduced up to five years and one month earlier.
The worst case has been less than four years for Leopard and every subsequent version. The Intel transition (Snow Leopard) had a very tight cutoff (three years to three years and seven months), but the best case has been improving since then, and is now just over five years for Mountain Lion. There is some hope it may improve further, unless Apple does arbitrary cutoffs rather than ones which at least have a technical explanation.
If you buy a new Mac late in the life cycle of that generation, you can be pretty certain that it will be able to run any version of OS X released within the next three years, and many (but not all) models will be able to run OS X versions released between three and four years later. Support rapidly dwindles beyond that, and it is rare to have a Mac model supported by an OS X version introduced more than five years after that model was discontinued or superseded.
If you buy a new Mac early in the life cycle of that generation, add the time that series is current, which might be as little as three months, but in some cases has been more than a year.
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These fake boundaries by Apple are really annoying. ... You also cant upgrade to Lion on the mac app store for some reason and have to call apple to get a key (seems really backwards to me).
Hiding Lion from the App Store is part of Apple's normal pattern: when a new OS X version is released, they stop selling the previous version through normal retail channels, but you can still order it (or earlier versions) by ringing Apple Sales. I expect this is done to reduce accidental purchases of an older version. If you have to at least talk to a sales rep, they can ask sensible questions to ensure you aren't buying the wrong version by mistake.
There was one recent exception: Apple continued to sell Snow Leopard on DVD via the online store while Lion was current, presumably to encourage Intel Leopard users to upgrade, but they did pull it from normal retail channels. Snow Leopard disappeared from the online store when Mountain Lion was released, but can still be ordered by ringing Apple Sales.