Wasn't the complete unviability of this extension put to bed in the 2002 TTC RTES study? -
http://transit.toronto.on.ca/archives/reports/rtes2002.pdf
It looked at 15 expansion options. Three were so poor, they were eliminated from further consideration before they even did passenger estimates. One of these eliminated options was the Sheppard extension from Yonge to Downsview. Another was Don Mills to Consumers Drive (and this was because they also included Don Mills to Victoria Park, as terminating the line and building a bus terminal at Consumers Drive was not practical). The only other expansion eliminated early in the study was the Bloor line from Kipling to Mississauga City Centre (because Peel and Mississauga weren't interested), however Kipling to West Mall and Dixie were carried forward.
The reasons the extension to Downsview was eliminated included the following
• Demand for rapid transit is considerably higher east of Yonge Street in comparison to west of Yonge Street.
• The high capital cost of such an extension is not matched by high ridership or re-development potential.
• Population and employment growth in the corridor is projected to be low.
• Densities in the area are projected to be below the threshold for implementation of rapid transit.
• The opportunities for feeder bus savings and commuter parking potential are considered to be low.
The remainder of the study was based on old 2021 employment/population predictions, which are now considered overly optimistic for that corridor.
So if even with the overly optimistic predictions back then, they didn't consider the western extension worthwhile, why do we suddenly want to consider it now. Particularly with the proposed Phase 2 Finch LRT from Finch West station to Finch station, which would surely siphon demand even further.