As for Mass Effect 1 remaining exclusive to 360 and or PC, yes that is likely to be the case unless a purchase of the published software rights takes place in favor of anyone wishing to bring it to other platforms, however, there is nothing stopping BioWare or EA from creating a "Mass Effect 1" based off another medium or even re-creating a new Mass Effect to take it's place from scratch.
Kind of like the LOTR, Tolkien's retainers can make whatever game they please based off of the rights retained, New Line and WB retains the FILM rights, and can create games based on the FILMS produced. Microsoft owns the rights to Mass Effect 1's software, yes the disc or digital format of THE SOFTWARE THEY PUBLISHED, they do not retain the rights to the IP of Mass Effect that BW and EA retain. This means only MS can reproduce that SOFTWARE, so a port is out of the question unless as I said there is a purchase of rights to reproduce or redistribute that software.
Legendary bought the rights to produce a FILM based on the Mass Effect IP, the film can follow the exact paths that ME1 took throughout the game, and thus a new MEDIUM is created, EA and BW can create and publish a GAME based off the FILM. Or like I said earlier CREATE Mass Effect from scratch. I'm tired right now but I'm not full of **** either.