Another reason why municipal boundaries are the worst places for high-density intensification,.... municipal infrastructure.
Since each municipality is responsible for their own infrastructure & services,... not just electrical, water, etc,.... but also schools (elementary to high-schools), community centres, libraries, police, fire, etc,.... these buildings tend to be located away from the municipal boundaries since they should be central for that municipal district (ie: stupid to put public school at school zone boundary) and local taxpayers really don't want folks from another municipality using their infrastructure and services paid for by their tax dollars! Thus, areas along municipal boundaries tend to already be underserviced by municipal infrastructures & services.
While some intensification of Yonge & Steeles is inevitable,.... with over-intensification of the area, it'll be the new residents that suffer from lack of municipal infrastructure & services.
You'll notice I didn't add Public Transit to list of municipal infrastructure & services since TTC run some bus lines north of Steeles subsidized by 905 municipality