The Nexus S launched in 2010 with 2.3 Gingerbread, received ICS 4.0 in the beginning of 2012 and JB 4.1.2. following that. That keeps it current up to where the HTC One was launched in 2013. 3 years of updates is by far above normal in Android and that's what we're looking for.
Assuming the Nexus 7 won't be updated this year or next year is just silly; the Galaxy Nexus went from 4.0 to 4.1 and then 4.2, based on the Nexus S it'd be safe to assume it'll be on KLP too. That puts the N7, which started on 4.1, onto KLP and the next two iterations based on a 3 upgrade cycle, and I haven't seen anything to suggest they're not trying to beat that as a standard.
Google actually seems to be doing the opposite, and trying to get all Nexus devices up to the newest build as fast as possible. Thinking about it in the terms of how Sammy, HTC and other OEM's treat their Android based mass consumer devices is a mistake. Google's money doesn't come from you upgrading, it comes from your participation in their universe as much as possible. The features to do that are on the latest and greatest software, not tied to any hardware.