What makes you think that it will take this long for there to be enough demand to justify a subway extension?
Look at the map of new residential developments posted on Steve Munro's website. Notice how many circles there are along Sheppard Avenue East and around Scarborough Centre. There are only a couple others areas in the city, excluding downtown, where there are this many condos being built (North York Centre, Yonge & Eglinton, Humber Bay Shores, Kipling/Bloor). Sheppard East is a hot area for condo developers; it makes no sense to build 20 and 30 storey condos and not build a subway. Remember that this is a map of proposed residential development from 2008-2012, so everything on this map was proposed or built after 2007, when Transit City was first proposed.
Then notice how few circles there are on Finch West. There are a trivial number of new condos here, but if you exclude Finch/Yonge, probably an order of magnitude less than Sheppard east of Yonge. Logically, Sheppard East ought to have much higher ridership than Finch West.