A large portion of the advertising is to get people to switch brands. A smaller portion is to get new customers. Banning advertisement increases the profits for the already established brands (for at least the short term).
Like higher quality / cheaper prices wouldn't spread by word of mouth... if anything advertising behemots are keeping actually improved products out of the spotlight by selling us whatever pays most in advertising costs, regardless of externalities.
I find that hard to believe, since most advertising is done by bigger brands.