I remember the arguments when WP8 was around the corner and people that had just bought the 920 were howling about it. Which is understandable since over the life of their new phones, some apps written for WP8 are now out of reach.
I thought then as I do now that MS made the right call on this. Whenever I consider my 1020 to be obsolete whether 6 mos, 1 year or 5 years down the road I'm glad that Windows Phone will have something viable to move into. Had they stayed with the old kernel to placate people that had just bought into high end models, Windows Phone would have been that much more delayed in their ability to move into the new processors and displays and would have been permanently shunted off as an unique quirky thing that didn't last too long...
I think I agree with others here who have stated that obsolescence occurs when production is no longer underway and the manufacturer stops supporting any given model. But to me, real obsolescence occurs when I as the owner no longer find what I own to be useful.
Surprisingly, the 808 was around 6 months for me before hitting obsolescence - and I think it arguably has the better overall camera mechanism. However, the email and internet clients on Belle through not being supported anymore fell so far behind to where they weren't very well functional anymore made the phone useless except for taking out for weekends - so the 808 became the same thing as my Canon PowerShot. The rest of the working week, I really didn't like using it and found myself missing the Lumia 900 as a phone. Once the 1020 came out I jumped on it and sold my 808.
On the other hand, I use and maintain two feature phones - Nokia 6610 and Sony T637 as emergency phones and some days I actually enjoy going retro and burning up some of my prepaid SIM that I have. The 6610 I rebuilt completely - there are still to this day OEM components available such as keyboards, LCD screens, batteries, etc. The T637 just took an OEM battery as my original just wouldn't recharge well (after a decade).
So in my experience I have two decade-old phones that can still take new parts that I find useful in a pinch and still enjoyable to use sometimes in their own right - so I don't really consider them truly obsolete. On the other hand the 808 which IMO could have been the top of the line in the industry with either Window Phone OS - or some kind of updated super Symbian if Nokia hadn't dropped that OS - became obsolete faster than I normally go through phones even though my primary consideration with any smartphone is the type of camera.
I guess everything hinges around defining what obsolete is for any one user..