The modern 'Urbanization Problem'.

The problem*: how to add additional rentable space in the way last disruptive to existing 'social contracts'. In my bubble, there are two general schools of thought: 'Missing Middle' (incrementalism) and Transit-Oriented Development (bounded high density) development. The first holds that we should permit gradual intensification everywhere, and the latter that we should cluster new high-rise development near transit stations. 'Missing Middle' affects a large share of the metro environment, even partial wins (triplexes) yield big changes. On the flip side, transportation research pretty clear that modest density increases (especially at the low end) don't yield any benefits in things like changing frequency, length, or mode shifts.

The 'Missing Middle' approach aligns with the small or non-professional developer, creating ADUs by 'barnacling on' to existing housing units. Strong Towns' is emblematic of this approach. What I call the 'TOD approach' largely aligns with the interests of professional developers, and often of city planning departments, as they both represent the focus of a finite amount of expert/professional energy on a limited number of sites, typically focused on created value or reducing 'blight'. The two have a 'bottom-up' vs. 'top-down' dichotomy, as well as a 'muddling through' vs 'technocratic' rationality. 

While I love density, I do not love its modern implementation, which largely consists of replacing defunct and vacant strip-malls along high traffic arterials with elevator apartments. As a resident of one, the noise is a misery, the traffic a barrier. The most affordable ones are the modern reincarnation of the railroad apartment. 

In contrast, there is little to be said against the Missing Middle, apart that it is feral and pernicious (and hence successful). It is also the heir to the near-totality of the human urban tradition (the pre-automotive part). 

*The historic 'urbanization problem' was how to deal with the health and safety impacts of crowding unprecedented numbers of people into unprecedently enormous conurbations, as highly paid factory jobs induced the rural population to become city dwellers. These problems mainly related to water: sewerage, drinkable water, and the nexus between them: run-off. But it also required enormous advances in transportation, including the oft-forgotten canal age, the railway age, electric traction, and the automobile. And indeed, enormous advances in structures: steel frame construction and the elevator. That the preliminary limit on urban growth/agglomeration has been regulatory is an entirely new problem that emerged within living memory.