This is precisely what happened. Quoting from my comment at the time (https://news.ycombinator.com/item?id=39579488) --
Subscribing to a pricing data service: legal.
Using data feed to guide pricing and set initial figures: legal.
Agreeing, either implicitly or explicitly, to let the pricing data feed set the price floor: collusion.
Leaving units unsold and refusing to lower the price despite insufficient demand: usually a bad business decision, possibly indicator of a market failure.
Leaving units unsold to maintain agreed-upon price floor despite insufficient demand: collusion, market manipulation. Congratulations, you are a cartel.
Almost always.
The trick is figuring out where the failure is coming from, and whether your society can find a lever to pull to fix it.
You're assuming they were the first kind of fools rather than the second kind.
> Leaving units unsold to maintain agreed-upon price floor despite insufficient demand: collusion, market manipulation. Congratulations, you are a cartel.
Which is the thing that doesn't work when many other landlords aren't in on it, because then they get to fill their vacancies while you're stuck with the empty units.
Notice also that antitrust prohibits attempts to monopolize. Fools are quite capable of doing something which is still a violation of antitrust laws despite being ineffective and unprofitable.
In general the homeowners are more numerous but the landlords are somewhat more organized.
The homeowners are also being taken for a ride, because less restrictive zoning reduces the value of units (since you can increase the supply of them) while increasing the value of each piece of land (since you can now build more units on it). And the homeowners (and small landlords) are typically the ones with a high ratio of land to units, but they get hornswoggled into demanding the thing that really benefits the corporate landlords who own the existing high density buildings.
Suppose housing units in some area are going for $600k but are currently limited to one unit per plot. That changes, now you can build up to 20 units per plot. A developer can build units for $100k/unit (their cost, not what they would charge you) plus whatever the property cost to begin with. In other words, they could pay you that $600k, spend another $1M constructing 10 new units there, and then sell them for $600k each, i.e. $6M from investing $1.6M. $12M on 2.6M if they build 20 units. What happens then?
They're going to start doing that whenever a house comes on the market, so over time the cost of a unit comes down, e.g. to $400k. But that's the price for the new units which are only consuming 5-10% as much land. Developers are selling those, not buying them. They want the land that doesn't already have a lot of units on it, i.e yours. And even at $400k, building 10 units gets them $4M from $1M + whatever they pay you. Which means whenever a unit like yours comes on the market, every developer is going to want it.
Even at the lower price per unit, and even if they had to pay you $1M for the land instead of the original $600k, the developers would still be making a tidy profit. And since they have to outbid each other when your house comes on the market, how much are they going to bid when not being the high bidder means not making anything?
Landlords have an inherent trade off between holding out for a tenant who would pay a higher rent and not having the unit vacant in the meantime. Holding out for longer than that helps the other landlords at your expense. Meanwhile there are a large number of landlords, there is no way for the ones using a given system to force the others to do the same, and a significant percentage of them don't.
So if the system tells the landlords using it to do the thing that would increase rents if every landlord was using it, the "defectors" who don't use the system win out over anyone using it, which means using the system rather than defecting would cause you to lose money and everyone would have the incentive to defect.
Whereas if all the system is doing is suggesting the optimal point in the trade off between holding out for a higher paying tenant and renting the unit sooner, without assuming the other landlords are being required to use the same system, that's the same thing an individual landlord would do if they're not colluding.
The actual way landlords collude is through zoning boards, because then they have an enforcement mechanism, i.e. if someone is trying to defect by adding new units to alleviate the limited supply then the other landlords can sic the government on them having passed the various laws against doing that.