That the dense city movement, of building up, instead of out, is ultimately ceding a huge proportion of our lives (our dwelling sizes and layouts, their materiality and designs, how the public space between them looks and feels, their maintenance and upkeep, etc. etc.) to soulless corporations trying to extract every dollar possible from us.
When we build out, people tend to have more say in the design and build of their own home, often being able to fully build it however they want because at a fundamental level a single person or couple can afford the materials it takes to build a home, and after it's built they can afford to pay a local contractor who lives nearby to make modifications to it.
What they don't have, is the up front resources to build a 20 story condo building. So instead they can buy a portion of a building that someone else has already built, which leaves them with no say in what is actually built in the first place. Ongoing possible changes and customizations are very limited by the constraints of the building itself, and the maintenance and repairs have to be farmed out to a nother corporation with the specialty knowledge and service staff to keep building systems running 24/7.
Yes, this is more efficient from an operating standpoint, but it's also more brittle, with less personal ownership, less room for individuality and beautification, and more inherent dependence on larger organizing bodies which always end up being private companies (which usually means people are being exploited).
In addition, when you expand outwards, all the space between the homes is controlled by the municipalities and your elected government, and you end up with pleasant streets and sidewalks, but when you build up with condos, you just have the tiniest dingiest never ending hallways with no soul.
And condos are the instance where you actually at least kind of own your home. In the case of many cities that densify, you end up tearing down or converting relatively dense single family homes into multi apartment units where you again put a landlord in charge, sucking as many resources out of the residents as possible. In the case of larger apartment buildings, you've effectively fully ceded a huge portion of the 'last mile' of municipal responsibilities to private corporations.
Yes, I understand all the grander environmental reasons about why we should densify, and places like Habitat 67 prove that density does not inherently have to be miserable and soulless, however, the act of densifying without changing our home ownership and development systems to be coop or publicly owned, is a huge pressure increasing the corporatization of housing.