Old Toronto for instance (and the 1950s suburbs) has a lot more missing middle than most assume. However these are often the first to go when it comes to new highrise proposals. From the developers stance it makes sense - we're talking about rental buildings on decent sized lots. Land assembly is much easier. But from a policy perspective it makes little sense to demolish a 6 storey walkup with a couple dozen units that while sometimes dated tend to be much more functional in layout. Also a lot more affordable.
Assembling land in SFH neighbourhoods is a tricky proposition at best when the average house is going for millions. We're seeing a decent number of multiplex proposals but these also tend to be in areas where the traditional housing makeup is/was already dwellings containing multiple units. Hence my assertations over the years that simply "building our way out" with zoning relaxations isn't going to work, even if it's a good thing from a city-building perspective. At some point you have to contend with the reality that growth is too high and concentrated in too few areas.
There's little value proposition in building a fairly simple development like this
https://maps.app.goo.gl/kjr6R1SR3kqQaNik6 even if it were more permissible.