First question: How long do you expect to keep the 920 before you replace it?
Second question: If you compare the 920's price and current features to competitive phones, does it best meet your desires now?
Last question: Looking at the possible updates that might come out while you have it, will any of those competitive phones be more likely to meet your future desires, or is the 920 still a good risk?
In my opinion, over the next 2 years the 920 is likely to fall behind the higher spec newer phones on firmware-enabled updates. We've already seen that in things like brightness profiles and the equalizer for the phone speakers not showing up in Cyan for the 920. However, if you compare it to lower spec newer phones that are priced about the same as the 920 on the open market, they didn't get these updates, either. Maybe they'll get something more in the future, but it's not a given. The 920 may stop getting firmware updates before they do, of course - that's the risk - but the question to you is will it still meet your primary desires up until you're ready to replace it?
As for OS updates, I think it has a pretty good chance of getting the next major update (WP 9 or whatever), at least in Preview for Developers form. I don't see it as being a lot less likely to get it than the newer phones that are currently at the same price level. Beyond WP 9, who knows? And again, the question is, if it doesn't get the next big update, does that mean it stops meeting your desires for the phone? If it does, then that's a risk factor; if not, then don't sweat it.
I bought a 920 in April after doing this sort of analysis. It made a lot more sense to me to get a refurbished 920 than a new lower spec phone for the same price, considering how long I expected to keep the phone. But this is your decision to make, based on your situation.