> When landlords threaten to raise rents in response to a future land value tax, why don’t they just raise the rents right now? Why do they have to wait until the tax is enacted?

I think the answer to this is because a tax increase is something they know every other landlord they’re competing with will be subject to as well. Raising it right now makes yours an oddly expensive apartment. Once the law passes though, even before implementation date, every landlord knows every other landlord will want to preserve the same cash flow they had before, by raising the rent by roughly the amount of the tax.

Unless of course you have mass vacancy rates already, and those rent raises would be more than the market can bear. But decent rentals in desirable cities have been in shortage for decades which is why rent steadily rises already. Even if your current tenants refuse to renew at a raised rent, someone else will come along, trading slightly down if necessary in terms of quality/amenities.