I think it would need to be over two years, which is generally the amount of time anyone might be expected to hold onto a phone that was sold to them. There really ought to be some more responsibility and/or accountability on the part of the carriers to support devices that know full well people will be more or less forced to carry for the next two years.
Which isn't to say that every phone within the last two years needs to get 5.0, but they should continue to receive updates and support as problems arise. My original Galaxy S (Epic on Sprint) had zero support on day one, but i think that was less about the phone and more about Sprint just sucking at knowing what to do with the recent Android explosion. Either way I still find that carriers and manufacturers all suck butt when it comes to phone support and I'll likely never buy a non-Google phone again. If they stop making them, I may just go to Windows.
Closer to the topic, I think most people who keep their eyes on the mobile industry know EXACTLY why adoption of 5.0 isn't happening. Only the Nexus line is getting the upgrade, and the upgrade runs like shit (and the G3 apparently has it). I own a Nexus 5 and 10. The 10 chokes to death on 5.0, so I leave it on 4.4.4*. My Nexus 5 had awful battery life, and busted wifi on 5.0, so I reverted to 4.4.2. Then when 5.0.1 came out I heard that wifi and battery life weren't so much of an issue. I installed the update and am pretty happy with it so far. I thought about grabbing a Nexus 6, but since they're impossible to buy unlocked without a great deal of effort and scripting I just DGAF.
So I don't know why anybody is remotely surprised by the low adoption rate.
*Some of the games I like to play just won't run on any version of Lollipop. I don't know if this is a Dalvik/ART thing or not, but the Nexus 10 just doesn't have the power for modern applications anymore, and I'll replace it within a year or so if some really great stuff comes out.