[noct]

Rebuilt from the core isn't very specific; the architecture has switched from PowerPC to x86. There are some solutions (emulation, secondary chipset), but none that aren't difficult and expensive.

As XNA is .NET based, they would only need to port the runtime libraries to the new native APIs.

They dropped support for the only SDK they offered that produced forward-compatible games.

Unless they announce a new form of XBLIG, the only way for indies to release games for the One will be via a publisher, and even with a distribution-only deal, it means MS is effectively enforcing a tax on indies that's paid out to AAAs.