That person needs to get their head screwed on. There's nothing wrong with cosplaying from any series just because it's ended. For one thing, they aren't considering that not everyone gets into a series while it's still ongoing. Does that mean they aren't allowed to cosplay a character they like? BS to that I say. (Is only just starting to read the Inuyasha manga; not that there's anyone from it I want to cosplay... yet at least)
Being an older/ended series doesn't mean it doesn't deserve some love. Quite recently, I located and watched most of the episodes of that Mighty Ducks cartoon that I had seen as a kid. I intend, once I have way more experience making costumes, to try my hand at a human-version of one of the Ducks. If someone tried to tell me that I can't because the series is ended, I'd tell them where they can shove their opinion.
Cosplay is about having fun dressing up as a character you like. If you don't like a certain character/ongoing series, why take the time/effort/money to make the outfit (barring if you're doing it because you're going as part of a group, that's understandable)