There are two types of updates which I think we're confounding: one is security updates, and another is OS upgrades. At least as of now, security updates are being forced through. So as long as you have a relatively recent version of Android and a phone with some sort of reputable brand, you get those regularly (just like your Windows/Ubuntu example).
On the other hand, version upgrades are mostly wild wild west-style of approach. And in this regard, it's a lot less different from Windows than you'd think (as for lowest-common-denominator-Linux in general, well, you don't see many good games on there, do you?). When Windows Vista launched, the minimum requirement was DX9. If you didn't have a DX9 card, you're SOL for the most part. But this is also the part where some PC hardware manufacturers have done very well (compared to phones) -- they provided continual driver updates. Can't say the same for phone hardware manufacturers (which is probably why the N5 wasn't updated).