I also wouldn't be surprised if Ho-oh and Lugia didn't use telepathy, and only made loud screeches, like Dialga and Palkia. Hopefully we do get to hear voices, since Lugia was talking in Movie 2.
That definitely was true for the 1st and 2nd generation of Pokemon, Kanto and Johto when the pokemon anime was really popular amongst kids and there was pokemon merchandising everywhere. I myself discovered the games through the Anime and so did many other people back then.
It probably was valid for Hoean as well. I'm not sure about now though. Most people in the 13 - 22 age bracket already know about Pokemon and the games and don't need the Anime to tell them what it is.
Whilst it's definately true game sales will suffer as a result , if that's offset by the reduction in costs of not having to produce an Anime then they'll go for it.