Increasing density also has a lot of downsides for the nearby residents. It means that congested roads become even more congested, and so on. Dirty roads get that much more dirt. Schools become packed such that students end up in temporary trailer buildings instead of classrooms. The problem is that all of these types of infrastructure tend to trail density, and often by many years or even decades. Seattle schools are physically running out of room to house students, causing programs to end and people to get shuffled around.

There are benefits to density as well, as most people who live in a city will attest. But there are certainly compelling disadvantages too, and I think any honest discussion about housing should recognize that residents of an area have a legitimate and fair desire to want their circumstances to stay the same or not get worse. Increasing the density of an area usually entails a lot of quality-of-life disadvantages for the current residents, by straining services and infrastructure, whereas it entails advantages for business owners and business developers and potentially new residents. It also sometimes results in lower housing prices, but the effect of this is not entirely clear (development is virtually never genuinely motivated by the desire to provide low income housing since that's a poor business proposition), and low housing price is not always a priority for residents compared to quality-of-life.

Some of the infrastructure challenges that I'm describing can be fixed over time, but it's by no means a given that it will happen at all: the lower quality of life might just become permanent. Plus shutting down major roads to widen them or increase throughput is a disruptive activity on its own, and is not always possible (where do you get property for a new school?) So major problems can trail increases in density by quite a while. Traffic congestion in particular can become considerably worse because of changes in density, and that takes a very very long time to fix on the scale of city planning.

It's also worth recognizing that people have different values, and so some people might value the increased benefits from further density differently than others, and the increased disadvantages differently. If you purchased a house somewhere that's quiet, safe, and has little traffic, because you keep to yourself and want to raise a family, then you'd be upset if the area around you was transformed into a college dormitory party atmosphere.

I'm not saying that I think there's a clear answer for how to handle city planning problems, but I think we could have better conversations about the problem by recognizing that development and density increases are usually bad for existing residents (those who can afford the area, are happy with the price, and value the quality of life they currently have). It is a conundrum to an extent that many of those current residents would not be there if it were not for previous decisions to increase density in that same city. However, this is only a conundrum on the surface: there is a tipping point where increasing density at first yields tremendous advantages but few disadvantages, and then slowly the disadvantages begin to mount up until they overcome the advantages. I believe many big cities in the US are at this tipping point, such as San Francisco.

Cities are democratic organizations that exist to serve their residents, and so if residents believe that policies are not in their interest, they will vote them down. These zoning questions are ultimately entirely political, just like the question of whether a country should accept Syrian refugees.