[ryandrake]

Sure you can. It just takes backbone, which is rarely found in the political class.

If I, as a voter, voted for a politician who promised to ban dumping mercury in the local river, I don't expect them to say "Oh, but any company already dumping mercury in the river can keep doing so, because we don't want to hurt people's livelihood." That's not what I voted for.

[MoltenMan]

Ok, but if you are investing capital in some sort of production line or industrialization you are not going to want to do that in an area where you might just lose your entire investment instantly; instead, you're just going to invest it in Texas or China. Of course with more extreme examples like yours you do have to put some cost on the existing companies to get it fixed, but it would be something with a smaller cost like having to dispose of the mercury properly (whereas in this article's examples they just flat out ban these things, which you can't do to existing factories).

[ryandrake]

For sure there would be a disincentive to "invest" in the area where you might lose the investment. That would be intentional. As a voter, I specifically don't want companies to be making those kinds of "investments" in my region. Go "invest" your dirty industry in China. If California's reputation for harshly regulating these things prevents these kinds of businesses from opening here in the first place, I consider that Working As Intended. We could make that reputation even stronger by not grandfathering things.