I'm using an '05 G5 tower, almost 5 years old now, and the only reason it won't last me another 2-3 years is the change in processor platform combined with a business need for particular software packages that aren't available for PPC at this point.
Since it seems unlikely that Apple will introduce another major architecture change any time soon (and since they'll be supporting the current architecture for several years afterwards), there's no reason your tower shouldn't keep going another 3-5 years.
Heck, I've known Macs that were still in everyday use after more than 10 years -- maybe not useful for the latest media editing at that point, but still in working condition and perfectly good for word processing and email and such.
Since it seems unlikely that Apple will introduce another major architecture change any time soon (and since they'll be supporting the current architecture for several years afterwards), there's no reason your tower shouldn't keep going another 3-5 years.
Heck, I've known Macs that were still in everyday use after more than 10 years -- maybe not useful for the latest media editing at that point, but still in working condition and perfectly good for word processing and email and such.