Problem is that NYCC will be even more difficult to commute to work on.There are only 2 things that would really help NYCC: One is when the downtown core proper finally caps out of available office space (which wont be happening anytime in the near future) or the second is when Toronto transit finally completely bursts from it seams and cant handle the immense load of commuters coming into the downtown core. The 2nd scenario is much more likely to happen first before the 1st.
Currently, firms are flocking to the downtown core and the only thing that would even give them a second thought is if they see that their employees are unable to commute to get into work. Only then would they start considering secondary nodes like Midtown and NYCC.
at the end of the day most peak period travel capacity growth over the next little while will come in the form of GO expansion - which services downtown. The DRL seems to (finally) be the next project on the docket as well, which also increases downtown transit access significantly.
At the end of the day downtown is so attractive for employers as it is the only place in the GTA that is accessible to 100% of the metro's labour pool. If you locate your office at NYCC you are restricting employee access to Toronto, York, eastern Durham, and western Peel. A lot of the GTA is too far and difficult of a commute to access it. If you locate in downtown, employees can come from Hamilton or Barrie or Bowmanville - the entire GTA is accessible within an hour or so.
One thing that may help is the Yonge north subway extension. It will improve transit access to the area, opening up a bit of a larger employment pool and hopefully marginally improving the attractiveness of it for companies.