So nuclear plants aren't needed, coal mining isn't needed, etc.
Coal mining for example.. all the tech that goes into developing that is no required anymore, or the requirements are very small. So these guys are out of work, the companies that sell them equipment are out of business, etc.
See where that goes? You end up with a lot of people without anything to do... There would be a halo effect from each business to each person. While in theory is may be a good thing, I don't think society is anywhere near ready for something like that.