It really just depends on what Microsoft feels like. Apple didn't know what they were doing, and thats why its easy to jailbreak. (Think XBOX 1).
Microsoft has already done all the research and learned all their lessons with the xbox 360. If they decide to apply that technology to the phones, than they will absolutely not be jailbreakable.
On the other hand, licensing fees for the 360 are much more lucrative than the mobile phones, so they might decide that it just isn't worth the 10-30 dollars per phone to implement something that sophisticated. In which case, they'll do something half-assed, and jailbreaking will be relatively easy.
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