Northern Light
Superstar
Very good questions! There was once a proposed Highway 48 bypass (sometimes known as Highway 448), but that was planned for an alignment further west. Traces of it can be seen in Malvern and Morningside Heights; it would have intersected the 401 at an interchange near Conlins Road, and continue north into Markham following 9th Line (the Donald Cousins Parkway follows some of that alignment). Apart from ROW protection, there wasn't any work towards it; that would have been a provincial project.
I've never seen any plans for major expansion of Meadowvale Road. The diamond interchange might have been built to accommodate traffic queuing into the zoo parking lots; sometimes it's still useful for that purpose despite lower zoo attendance. Traffic can easily be diverted to overflow lots by staff or police, better than if there was simply a left turn into the main entrance.
Highway 448, or variously the Morningside expressway was to follow the Morningside Avenue alignment/proposed alignment which would have skirted the western side of the Rouge Valley.
I can't see any obvious reason for the interchange other what you have postulated.
It's too far south for a straight-line Finch that was once contemplated.
I can't see an obvious east-west corridor than Zoo Rd. might have been plugged into, nor any use for further widening/grade-separating Meadowvale.