We are getting into some serious topic drift here...
Anyway, having had some time to observe the problem of backwards compatibility from the many perspectives, we all should take note that there is almost nothing in it for Apple or your favorite software developer to keep backwards compatibility beyond a certain time frame. Some examples:
From the developers' perspective -- Sometimes Apple introduces some new feature that makes life much easier for the software developer. However it is only on the last two major OS's. The software developer is then looking at making a choice between writing two lines of code to call the new API, or spending a week or more reinventing that wheel so that 2% of his installed base can play the game. There are usually several such. Is it worth your time to add another month or two to your schedule in order to service that 2%?
Another from the developer's perspective -- lets say that MacOS X.4 (Tiger) has a really annoying bug that causes your game to go haywire in a way that is difficult to track down. And as is typical you no longer have a machine that will boot on the Tiger (X.4.0) install CD, so you can't reproduce it. Are you really going to go buy a system on e-bay to support these users?
From the OS providers' perspective -- it is a lot of work at times to support diverse hardware. For example while we were on PowerPC, you often ended up writing high performance OS code twice, once for G4/G5 and once for G3, which didn't have AltiVec. Likewise, you can get into similar situations for DX9 vs. DX10 vs. DX11 compatible GPUs on OpenGL, etc. Instead of using a single core piece of code that everyone runs, you now have a rats nest of code designed to handle every possible situation on a wide variety of GPUs. After beating your head on that for a while you really begin to dream of the day when you can finally retire "bad idea X" hardware and no longer have to deal with the associated headaches.
From the OS providers' perspective -- Microsoft has a big problem of too much backwards compatibility. They have to compete with the Microsoft of 10 years ago (WinXP) in order to get new sales, and quiet frankly they aren't doing that well. If the customer really wants WinXP, it is hard to convince him that Win7 is more compatible with WinXP than WinXP was. You eventually convince yourself that you can't actually fix bugs because the fix might break someone depending on the bug. At that point, your OS stagnates and you have trouble delivering new stuff. You pre-announce a whole pile of new features to get people excited. Then you go implement them, discover that it breaks must-not-break-apps X,Y,Z. Then in despair you pull the feature. At the end of the day, you over-promise (longhorn) and under deliver (vista) and nobody is happy.
In any case, if you are depending on Dad to give you a hand-me-down computer so you can avoid falling off the OS bandwagon, then don't whine about how poor you are. You should be busy convincing Dad that the latest and greatest mac is the best thing ever and he really needs one! It is his birthday soon. Maybe Mom should get him one. I'll show you just the one. Or maybe it's Mom's birthday soon and she really wanted that MacBook Air. Chances are, no matter whose birthday it is, it is you who will be getting the present -- a nice NEWER hand-me-down computer! You'll need it for college anyway, I'm sure.