It might seem reasonable to you, but the argument that the supporters have put forward in favour of this plan is that relocating jobs to Scarborough will somehow attract the private sector to do the same. That is not gonna work:
Indeed, the assumption that relocating the government jobs will prompt the private sector do the same is dubious, or at least not supported by any stats.
I see other benefits in relocating those jobs: a bit less crowding on the downtown subway lines, and more job opportunities for the locals.
Sure, if they live in the East end (as you pointed out). I wonder how many west end residents would agree with that. Or people who rely on GO transit. For every person that this would benefit, somebody else will be mightily inconvenienced.
Most of people are partly mobile in their jobs. Not able to switch jobs immediately, but can find a job in a more convenient location after a period of search.
In that view, having the government jobs somewhat distributed between multiple nodes is preferable to concentrating all of them in downtown Toronto.
For those people who prefer to work in downtown, options are still plentiful. That's the largest employment node in GTA, and will remain the largest in the foreseeable future.
For the record, I would be just as happy to work in North York, if it's on Yonge St. My commute would be even easier. Whereas to get to Scarborough Centre I would have to take a bus, subway, and then another bus. Cycling is not an option as the route is too distant, hilly, and unsafe. I hope you also realize that most office buildings in Scarborough Centre are not well served by the RT to begin with, and will be even less so with the one stop subway. A lot of people may face a 15-20 minute walk, or have to transfer to a bus.
I think that 20 min is an exaggeration. STC is bounded by Brimley, McCowan, Ellesmere, and Hwy 401. That's a rectangle 800 m wide and 1,200 m long. Three out of 4 corners are occupied by a parkette, a 401 ramp, and a supermarket, thus not suitable for office jobs. Assuming that the subway station and the bus terminal remain close to the middle of the area, most of office buildings should be within 1,000 m walking distance from the station. That's a 12 min walk for an average person (5 kph).