Sure there is. It's 1.7 km from Scarborough Centre to Sheppard. if subway instead goes up Markham to Progress, it's about the same length as the current proposal. Better yet, go north on McCowan, then turn at STC to Centennial. Same length, but serves Centennial instead of McCowan Sheppard.
I don't recommend this ... just saying that you can serve further east at a similar cost, and hit STC.
Obviously an SRT replacement that misses the SRT and STC completely is dead in the water, even if it does make more sense from a subway ridership basis - simply because you've still got the current aging SRT to be dealt with.