Last I read none of the closed plants in the USA had been completely decommissioned.
Much, if not a lot of this has to do with the simple fact that after you mothball the plany, it's easiest and cheapest to wait a while for various radioactive isotopes to decay to less hazardous stuff. As long as you mothball it properly, and maintain that, there's no hurry.
If the money is set aside ahead of time and invested well then sure. If your dependent on the company that operated the plant to still be around then there just going to keep putting it off until the company fails. At which point the taxpayer is stuck with the costs.
Much, if not a lot of this has to do with the simple fact that after you mothball the plany, it's easiest and cheapest to wait a while for various radioactive isotopes to decay to less hazardous stuff. As long as you mothball it properly, and maintain that, there's no hurry.