There are plenty of railway bridges that have been repurposed for use on a pedestrian or cycling trail, so it's certainly a possibility.
What we don't know is whether ML does have a long term intention to use that bridge for some transit project - eg VIA HFR, or in conjunction with reactivating the Leaside Spur to reroute the Richmond Hill service away from the Don River. (I see the latter as a very long shot, as the NIMBY reaction would be intense.... but it's an option that's on the table in theory). (PS: If Pickering Airport ever were built, an extension of UP on this route might be considered....)
I expect they would not be eager to "temporarily" repurpose it, because that would invite a hue and cry when they tried to take it back for rail use. It's understandable if they prefer to let it sit idle until their plans firm up.... which could be decades.
But if HFR is going a different route, and the Leaside Spur is truly gone..... and the bridge is not needed for transit.....by all means it would make a great addition to the trail network. Given the environment, and its length and height, it would need proper siding and flooring to be safe.... but it's fundamentally good to go in that capacity for another century. Maybe it's a good opportunity to push ML to fish or cut bait.