The life expectancy of an analog receiver is about 30 years give or take (also dependant on use).
Ok, well no one really knows what the life expectancy is but realistically these electronics could last you a lifetime.
Now if you are referring to the technology onboard the receiver, like the all important DAC for Dolby and DTS, well that expectant lifetime is anywhere from a couple of months to several years i would say. Some of the new, higher priced receivers are offering computer connectivity for updates to the software/firmware of the DACs onboard the receiver.
Even still, 5.1 today should do you just fine for a darn good long time. Heck, there are stereophiles out there who still snub their noses at the mere concept of surround channels and stereo has been around for...how many decades now?
Do not be so concerned with what is in style and what MIGHT be in style in the next xy period of time. Something you buy today will always be out of date by next year. Digital vs. analog receivers? Right now alot of hype and questionable as to the enhancement o signal it will bring. The terms 'upgrade' and 'better' are often abused as adjectives in this audio realm.
And that is the rant for the day.
