Titles have different publishers with different goals and they have to test each title and make sure each game works as expected, based on that they are not going to commit to anything they don't have approval for. Making a sweeping statement is only going to get them in trouble.
Phil has mentioned past GwG titles, I am sure he will be working to get them on it but guaranteeing all of them is probably not something MS can do as it is.
Uh, I'm only talking about "from now on" which is what he said. This is "from now on" now.