Considering the focus on fast on-foot movement, wall climbing and so on, mounts would undermine the feeling of freedom and speed they've been going for in the same way that sticking Sonic the Hedgehog in a car would.
nothing really isn't stopping sega from using the mount idea tbh. they could just use it as different form of photon dashing like they let you choose your photon dash in the salon. all the mount ideas have to do is follow the mechanics of the photon dash.
i think someone thought of a photon dash idea where the player is carried by a group of rappies.