It's true that Nako might end up coming back only as a customer, depending on how her life develops otherwise. It's true we saw her teach swimming, but that's something quite a few students do on the side, earning some money to support their school lives. However, how many can actually do it for living later, is another question, especially in a country like Japan with a declining population. Realistically speaking if she manages to pull that off, I wouldn't expect her to return to the inn anymore as a worker. No matter how much she loved the place, for a young person in a few years it will feel more like a fond memory than a future career choice.

For the others it's an entirely different thing, though. They already were doing the very thing they wanted to do at the inn. So, for them it could be returning to the ideal workplace. Although with Tohru it would depend on the position he had advanced to. He might not want to return to be the second cook if he had already gained a higher position elsewhere (that didn't suck as a workplace either).