Good points.
I think Yonge should be reduced to a single, ample lane of traffic each way with the sidewalks widened to make up the difference. I'm not sure how much tree weight the subway roof is or could be engineered to hold, but there are plenty of types of trees that could provide good visual lines and cover while working within it. Put down real pavers, put up better street furniture to go with the lighting, and get the goddamn strip properly lit between Bloor and Wellesley. It's like a forgotten corner of New York City circa 1977 up there. Actually, a friend up mine up from New York who used to live here commented - "What's up with Yonge street up there? It looks so...fetid?". I agree. Charming, tumbledown, homey...these are one thing. But dank, low, scuzzy and underlit are quite another. With all the massive increase of density in the last ten years throughout and around this whole area, and the resurgence of interest in automobile-free access, the increased and lasting pedestrianization of Yonge is an idea whose time has come.
As for the decay and lousy sloppiness, it happened as Yonge went from being Toronto's only main shopping street - or only much of anything - though it was neat and tidy, down through it's evolution as a 'sin strip'. Sometime after that it stopped being funky and just got lazier and grottier as the city spread out. Everything used to be centered on Yonge - gay, straight, seedy, respectable, accessible international restaurants for tourists, the works. But much as the Church/Wellesley village has become grossly lax and seedy without pressure from the outside pushing it together, so with Yonge. There are so many other places to be and things to do that it really hasn't had to keep itself up. That's a loose idear, anyway. I wonder what rents are like per establishment, what's included or not by way of building repairs and upkeep from tenants, how many landlords are local, etc. We've seen the cost of distant and negligent landlords in the disgusting loss of the Empress Hotel. I'm sure financial conditions on Yonge vary a lot, but there's got to be something more than just lax attitudes or landlords waiting for demolition to make money, to account for it's shabby state.
A small part of Yonge Street's problem (and a widespread Toronto 'thing') is it's vista-less non-ending linearity. The street just goes on and on. Not a bend, not a split, or monument or traffic circle. It's gentle consistent slope makes for a peculiar slow run downhill on onehand, and an almost subliminal increase in difficulty the other way. It doesn't terminate well at the lake in a way that honours it and makes it visible when you're blocks north, and northwards, there's no convenient hill or obstacle to get around that might give a decent sense of closure visually. The street just seems to drain on forever, rigid, one way or the other.
A city critic once noted that Toronto's best walking streets are usually east/west instead of north/south. I wonder if the insinuating, almost unnoticable rise (or slope down) of the street has anything to do with this.
Ideally, one could look down Yonge and see a clear opening to the water. The Gardiner and the tracks have made sure that won't happen, but at least one end would have a wonderful vista and sense of arrival. As Jane Jacobs said "There must always be an end in sight, and the end must not be final".
Personally, I wouldn't mind seeing, say, the Yonge/Wellesley corner undergo some major surgery (buildings moved entire, etc.), and something along the lines of a traditional 'round with a typical fountain, obelisk or statue setup go it. Just to keep things humane. A friend of mine has always said "there's something wrong with Yonge Street", referring to people hurrying up and down the sidewalks, up against it's walls, with little sense of leisure or enjoyment.
It's not a leisurely street. That's a problem. It needs generosity, and not of vehicular traffic. It needs greenery. Look at any street before and after trees. All the bemoaned polygot ugliness of Yonge would disappear as if into a mist if the sidewalks were enlarged, and green trees were planted from Lakeshore to Bloor. In fact, it's chunk-a-lunk complexity could become an instant benefit. As it is, it's all we have to look at, unflitered. A half-decent veil of tracery would go a long way. Leaves in the summer and fall, lights in the winter and spring.