Considering that Sheppard was such a costly mistake, one could look at its grossly enormous capital and operating costs as stealing funds from the rest of the TTC.
And even if an extension instantly resulted in a 50% increase in ridership, it would still be an over-subsidized financial burden that drains (or steals) funds from elsewhere in the system. More crowded buses for Finch East, Finch West, Don Mills, Dufferin, Eglinton West, Jane... But fret no more because we'll have a near-empty suburban subway line to a Scarborough shopping mall!