It sounds like they lead a very good hermie life. The thing with hermit crabs in captivity is...they're all wild-caught. Because of this it is impossible to tell if they are carrying inernal parasites or something else that could be harmful to them down the road. The captive lifespan of a hermit crab isusually between 4-8 years.....but in the wild they often live to 75 years or more, so there is something that we all are lacking about taking care of them in captivity.

The only suggestion I would make is to add a little bit of meat to their diet, in the form of canned dog/cat food.

I don't know that your setup looks like, but just in case....Make sure their sand is deep enough for them to burrow into.