Two separate SKUs for streaming makes little sense, it would be much better and simpler to have two tiers of streaming services. More profitable, even, I believe.
Unless they think it would drain from the perception of value of the 1080p physical console, so yeah, perhaps they're thinking about competition among their own consoles.
Edit: simpler yet: slap the next-generation streaming service into the Xbox One as-is and call it a day. Sony could do it with the PS4 I wonder.