What you're describing is the inability to maintain quality over a career, which is a separate issue from whether auteurs exist. Often it's because fame and success remove the constraints that were forcing the work to be tight and focused. I don't think the idea of a game auteur is any less real than that of a film auteur. Both are made by large teams, but the best works are often a single person's vision.
An auteur is just the "author", the primary creative force behind a project. Sustained success is orthogonal to that and depends on the person. Francis Ford Coppola didn't make anything of note after the 1970s, but he was still the auteur behind The Godfather and Apocalypse Now.
