Access to the data as currently provided on the site is free and will continue to be free. If the site becomes too expensive to maintain the data will be given to someone who can continue to provide it for free.
The model for the transcription project relies on everyone deriving a benefit. Everyone benefits from free access to the index and that is what motivates volunteers to provide free labour. It will never be in our best interest to try to revoke access as that would make us a pariah in the genealogical community. We think the 1901 census index project is an exciting illustration of how internet technology can allow people to cooperate and collaborate on a scale that just wasn't possible in the past.