uhhh...Toronto has plenty of examples of streets lined by 1-3 storey buildings. The difference is that Yonge is arguably the most important street in the city. Show me a picture of Tokyo's most important streets, and I guarantee you that they aren't lined by 2 story dilapidated townhouses. Even the cities that value heritage the most, in the European Union, had the sensibility to modernize their main business areas (ex. La Defense, Paris). Nobody's asking to tear down St. Lawerence here. The fact is that Yonge isn't as glamorous as the historians on this site make it out to be, and I'm not only referring to it's current form, but also it's potential with the current massing. It's sad when the city's most important road becomes an area to avoid when showing out of towners around, and I know I'm not the only one with this sentiment.
I know this has been pointed out countless times to you but I couldn't resist, you seriously bring guests to Yonge street ? Why ? I never do, I never recommend it, I'm referring to the part a little bit south of Bloor to about College / Dundas. Why bother ? There are so many other interesting places in Toronto to visit, the only apeal of Yonge is its proximity to the core.
Anyway, I've been noticing, slowly bur surely, Yonge is indeed gentrifying, more restaurants / a couple retail outlets, and quite a bit less dollar / souvenir / cheap clothing outlets.
So I'm sure, 20 years from now it'll be very different, whether you bring a guest there or not is dependent on what they like. So really the only question for us to answer is what sort of street should it turn into. Both in terms of the built form and the type of retail outlets that populate it.
Have you been to Oxford / Knightsbridge / ... instead 2 or 3 other famous Long streets here (many consider the pinnacle of shopping in London for the typical middle class tourist) .... they are nearly identical to each other, and less the architecture, its no different then the Eaton Center ... and again these streets almost mimic each other in terms of the stores and the ordering of them. Its actually very entertaining walking these so called great streets just to find out how boring they are simply in terms of the type of retail that exists ... btw that's not to say London has many more interesting areas, just like Toronto does.
Is that what we want for Yonge ? Personally .... sure ! I wouldn't mind a tall, Queen east of Spadina is already like that, why not have a place to shop other then the Eaton Center, and again, Toronto has a lot of other what I'm sure most of us would consider interesting streets. Honestly to me Bloor street is no different, other then the fact there are many high end shops that I'm sure some find interesting it has no apeal to me, whereas Yorkville proper is different due to the charming nature of the streets / alleys.
So I say, make Yonge == to Bloor, not high end, rather chain retail, and a very well manicured street. Essentially something the typical boring tourist will like. I honestly find tourist who want something off the beaten track (to a certain degree i.e. the Queen W / Kensington / Junction) could careless about the built form.