While this is partially true, various other teen hero teams managed to work just fine after the Legion's heyday, including the Academy X years of in the X-franchise, PAD's Young Justice, Image's Gen 13, the Runaways, Young Avengers, Avengers Initiative, Avengers Academy, etc. All had multi-year runs and iddn't all just 'fail because of low sales,' but often ran their course because the writers were done with the stories / arc they were telling with those characters.
(Granted, there seems to be a whole lot more Marvel on that list, but DC *should* be able to match that with teen hero teams of their own.)
I strongly feel that 'teen heroes don't sell' is less true than 'DC gave up on any lasting book that wasn't Superman or Batman (and I sometimes feel like they don't really love Superman...).'