It's possibly the case that Apple's lack of upgrade support is a feature, not a bug (though it introduces the complexity of eventual lack of security support).
1) It's up to the user to decide whether a machine is usable for their needs or not, not Apple.
2) Locking users in at an older version and then refusing to support that version even just 3 years after it's come out is just too short a time window.
3) Offering no way for users who need an old version to get it except illegally is a non-starter for any purpose, security or not, again, because 3 years old is just not old enough to so completely phase it out.
While I agree that lack of usability is a concern, it seems to me that between lack of upgrade-ability, short OS version lifespans, and early phaseouts of old versions, Apple is intentionally forcing users to consistently upgrade once every 3-4 years at most. Fine business model, terrible for the end user, and a real detriment to Apple for me personally when the competition doesn't behave that way to that extent.
Well yes, but it is also a business decision. Further supporting an old machine with a changing toolchain means additional costs for development, testing and troubleshooting. Those are costs that Apple has to bear.
I won't contest your point that 3-4 years is too early, just saying that there's more to supporting old hardware than just including the same drivers as in the last release.
Mavericks introduced RAM compression, so for RAM-constrained machines it may be even better than the previous OS.
It actually runs great. I'm using a 2011 Air w/2GB (cheapest back then) and El Capitan and it's almost as fast as my 24GB Mac Pro.
I push it a bit with Logic Pro X + drum samples (for my V-Drums), but it handles that without complaints. But for web, videos and programming there isn't much of a difference.