For the Stouffville corridor a lot of the track has been laid for years. They could have opened it in sections but they never did. The section from Kennedy to Steeles could have been opened a long time ago.
Not without the West Highland Creek bridge and the stations, it couldn't.
The work was oddly sequenced, since the track contract was placed first and is largely complete - but without the platforms being ready at Milliken and Agincourt, and the bridgework at West Highland Creek, all that new track and sound walls and grading have not led to any new capacity.....yet.
Something had to be finished first, I suppose... not good planning nor good cash flow to delay it all until the last critical path item gets done.
I'm sure there is a back story to why the creek bridge has taken this long, but with the station work plodding along perhaps it has always been moot. There was talk that the contractor wanted work windows for the bridge, but ML was not in agreement - it's likely proven to be a pay me now, pay me later proposition with ML having no choice at this point but to allow them. Let's hope that last bridge now gets done promptly, as the stations appear to be close to completion. Oh, and let's hope that the added service doesn't have to wait for track capacity west of Scarborough Jct is restored as the Ontario Line work gets done.
I especially agree with @Northern Light that regardless of the past, at this point there ought to be a detailed and transparent explanation of the timelines and milestones towards 30 minute or less service.