When companies started creating computer software, they realized that, in order to have more control over their products, they made licensing agreements. So what happens when you "purchase" a game? You purchase a copy of the software and have a license to use it. Its simple contract law. Unlike music (when it was purchased on vinyl, tape, 8-track, and I still have a few of those) purchased in the past, it is easier to file lawsuits with a licensing agreement. It makes business sense, considering how relatively easy it is to manipulate the program. For audio in the past, unless you had access to the master recordings, you really couldn't accurately copy the sound, as artifacts would give away the copy.
I have faith that Microsoft will consider backwards compatibility. I'm sure they will also maintain the Gamerscore as well. They are pushing Microsoft accounts and have the ability to award achievement points across many platforms. But be mindful that their bread and butter comes from the sale of games. Therefore, I'm of the opinion that there will be a fee associated for transferring licenses.