I had a used Cannondale road bike for about 3 years when I was just starting out. Jumped up to a beautiful Kestrel road biek that didn't fit. Road that for 2 years then got my wonderfully comfy QR in 2003. I have no reason to replace her other than being wooed by all the shiney new stuff, which isn't very swaying to me.

I'd say 5 years on a road bike is more than enough time to determine if you are serious about triathlon. Go for it.