Okay.
Imagine you have to produce a single level, it's one building wide, and it covers ten buildings long. Mirror's Edge's 'tracks' are like that. The density of the design allows for multiple paths through those, despite being very geometrically narrow. If you remove all linear limitations, and make it basically like first person Assassin's Creed, it's just not possible to design every possible path with the minute detail of the tracks in the original.
That's not really a problem if you don't care about the time trails, because you don't need to pay much attention to the design to find shorter paths, as you'll just run a single mission once, but I personally don't care about the freedom of being open world, lots of games do that, no other games do what Mirror's Edge did. Truly 3D platforming.