I think the subway is on the 30 year+ plan, so perhaps they do what they can now but acknowledge that it's probably the next lifecycle of Stephen Ave that will warrant more serious consideration of a subway. I wish it was sooner - I want the LRT to convert more and more to a grade-separated light-metro style system, rather than the way it's going with more at-grade crossings added slowing things down. Calgary at 2.5 million is going to need the Red Line to do a lot more than it currently can (as impressive as the capacity is at-grade today).
Back to Stephen Ave upgrades - I think what's often forgotten here is the project is really is mostly utility work in cost and time. It's truly an iceberg where 90% is below the surface - all the cost, delays and risks are beneath the ground. It all compounds, because not only do you have to dig deep, you have to keep access open and keep everything safe - safety fencing, supports, openings, closures. Digging underground will inevitably hit something unforeseen, delaying things further. It's so much harder than just the surface treatment.
If they didn't need to upgrade any pipes, they could probably complete just the surface job in a month or two. That's just scraping off the top layer and putting in some new pavers and light poles. You'd hardly need any fencing or closures beyond some temporary stuff for accessibility - there's no big excavations to fall in.