Mills are important artifacts. A museum is just a building which contains artifacts. Mills are more, they carry information about history, change, people, and in the process give a better understanding about ourselves. Mills are very costly to restore. What most often happens is a group gets the wrong understanding, once you have put the big bucks into a mill restoration, you never have to do that again.
Mills require a lot of attention and maintenance.
The best way to preserve a mill is to operate it, and the more it's operated, the better it's maintained. Some mills that have been restored may now operated one or two days a year. Mills are affected by seasonal and climatic conditions, they were never intended to operate on an off and on basis, which ends up being mostly idle. Another mistake groups sometime make is after they have spent big bucks restoring a mill. They think people love mills so much, that they think someone will operate the mill for little or noting.