But why not? One role on one show, movie, no matter how beloved, is in no way an indication of a career.
Look at Summer Glau - she started out as a ballerina who didn't even have lines on Angel and went on to star in Firefly, The 4400 and T:TSC and that are just the ones I could think of off the top of my head and I highly doubt that's the end of her career.
Let's take a non-Buffyverse, really high profile example: Johnny Depp had a role in Nightmare on Elm Street where he had about two scenes and killed off quickly. Going by the logic you're using, if you were a NOEM fan, that boy couldn't have gotten too far, surely? And, well, I don't think I have to spell out his career.