Probably the biggest externality here is the environmental cost of producing all the single use plastic containers in the first place, but the landfill itself will have externalities too.
Eventually, the rate of semi-permanent waste creation by humans on Earth will have to equilibrate with the rate at which semi-permanent waste is absorbed by the environment. Otherwise, the amount of waste would continue to increase indefinitely. We can certainly go a very long time at our current rate of waste creation, which is why the costs to future-people isn't included in the price. But those costs are real, and potentially include making sure chemical leeching doesn't occur over hundreds or thousands of years, not just decades.
Such as? As I mentioned, landfills are already regulated to prevent leeching harmful byproducts into the environment and many of the harmful things produced by landfills that do escape, e.g. methane, isn't really specific to the landfill itself.
Your entire block might switch to zero waste and would likely see no reduction in their property taxes (any reduction would probably simply reduce the local government's deficit).
Where is this true? In every jurisdiction I've ever lived in trash service has been private and I've had to pay for it myself. The service fee is based on volume of trash container. I've always had direct feedback on trash disposal costs.