The cost of cleanup typically falls upon the person who builds there next.
If that new Walmart is built on a property that already had a strip mall, first they tear down the strip mall and bear that cost.
The new Apple campus is a great example. They tore down a bunch of old HP buildings and had to bear that cost. They reduced the cost by grinding up the old buildings to make concrete for the new building. Also, they probably discounted the purchase price of the land to account for the cleanup, so in some respect, HP bore that cost too.
Yes and that cause issue of abandoned sites, if the desirability of the site is lower than the price of cleaning up. Worse, there is not necessarily negative consequences that would push the market to avoid such situation. For heavily polluted sites like gas station, chemical plant, ... the owner of the land has generally made a profit, so disposable land is a viable business model. Not even counting that after enough years, the site could look clean enough to be resold.
At some point, it was discussed in Europe to impose a viable (i.e. funded upfront) reconversion plan for land used for stuff like gas station or landfill. The trigger in this case was some incident about some block of flat built on some forgotten nastiness by an unscrupulous builder. Not quite sure if anything actually happened.
If you are worried about the Sun dying in a few billion years, then yeah, you are technically right.
And yes, the entropy of Earth is decreased by cleanup efforts. Only when including the Sun you get a system in which the total entropy increases. But this would be quite useless thing to do, given that the Sun is an infinite source of energy for practical purposes.
In more concrete terms: if you pick any given point in time, you'd probably be able to say the same without being wrong. At date X, the social system in practice is/was the one which worked better up until that X point; it's because we've been evolving somewhat continuously, not because the system per se.
Observe any abandoned or unused commercial structure in your town, such as the old building Walmart vacated to build a bigger Walmart 1000 feet down the road. Did the price of building either structure include the cost of cleaning them up?
Thermodynamics guarantees that literally everything we might try to do makes a mess (a net entropic gain), and that includes cleanup efforts. Cleaning up is really just shifting messes around. We spend a little extra energy to make a neat pile but at the cost of producing a little extra poop and carbon dioxide.
We kick the entropy can down the road a thousand different ways every day. And then we do things like shipping cotton across the whole god-#@%^ Pacific ocean, so Chinese slave^H sweatshop laborers can assemble them into clothes, so we can ship them back across the ocean again and buy shirts. The ludicrous inefficiency of our corporate masters sacrificing both natural resources and our country's prosperity to save a buck is conveniently abstracted out of sight by the glorious global capitalism.
Make no attempt to look at the man behind the curtain.