I really do wonder if the person had an ulterior motive telling you that...
Like, I could understand the argument ("You wont get enough attention!" - eh, some people cosplay for attention, whatever) if we were talking about, say, something like Urusei Yatsura, or Cat's Eye - y'know, stuff that the new generation of fans might not know or recognise at first glance. But for something that might end in the near future, that's ridiculous.
Not to mention that there will be probably still tons of MLP cosplayers for quite some time, so it's not like doing a popular series is a surefire way to get a lot of attention.
Even then, lot's of old series are still popular and how would that argument even hold up against movies or video games? Those things end right after they're released, so by that logic they would never cosplay anything from these?
There is a difference between being relevant and still running.