It would backfire because it would show Toronto as being so desperate to push out a rapid transit line, and look good, that they did so before finishing it. This isn't some non-place like Oakwood or Laird we're talking about here, it is the main access point for thousands of people from midtown, it is the crown jewel of the project, and putting it into service without the station being ready would be like putting a new train into service without seats.
Plus, how would you serve the people living in the Yonge and Eglinton area? They would either have to take the bus and transfer to the 5 elsewhere, which would be a really bad look from a PR pespective, or you'd have to continue running the overhead bus service in its full, present day configuration, rather than the limited scope it will run in post-line opening, which would be a waste of money.
Let it stand. We've waited more than a decade for this, another year or two isn't the end of the world.