I don't think it can be blamed on Google if phone manufacturers don't update their devices fast enough and if Google would take stronger stance in OS updates and manufacturers would jump ship wouldn't Microsoft face same issues?
Or did I miss completely what you meant?
In Android world, just like in the old days of Symbian, Series OS and other OSes, updating means getting a new phone.
You aren't even safe with Nexus if the manufacturer jumps out (TI).
Google apparently didn't had any contract in place for forcing TI to keep doing the updates in case of such situation would occur.
Neither they thought it was worth to spend some Google resources doing the updates themselves.
So they went to third option, letting their Nexus customers that they aren't any special and better buy a new Nexus.