Why would it go prohibited? The only real reason this could be prohib is being a current prohib variant or being easily converted to full auto. This is clearly a new firearm, and not a variant of something currently on the prohib list. I also trust that the highly experienced engineers at sig have made a firearm that is NOT easily convertible to full auto.
It does have some similarities with the AR... But so does the XCR. Being deemed an AR variant would be the only reason why the MPX would be deemed straight restricted, we will likely see restricted and non restricted versions, based on barrel length.
The MPX is currently being offered for preorder in the US from Rockwell Arms. Preorder price is $1999, with 6 to 12 month delivery expected.... In gun months, that's probably 12 to 24 months. Add 2 years for classification in Canada....