WislaHD
Superstar
People going west from Scarborough, I think, don't care whether they're going along Eglinton or Bloor/Danforth. They're likely going to transfer at Yonge anyway. This also has the benefit of moving the traffic out of the bursting Yonge/Bloor Station.
It is pretty foreseeable that Yonge-Eglinton will become the second choke point of the system once the Crosstown is complete. I'm betting the experience for Scarborough commuters will be the same whether they transfer to Yonge on Bloor or on Eglinton.
Except Eglinton will likely be slower & less frequent than Bloor-Danforth above ground, and commuters will still have to travel between Eglinton and Bloor to their downtown destinations.