I recommend 'Second Honeymoon' by Joanna Trollope, which reflects exactly this situation.
The parents in the book have three adult DC who have left home, but then want to come home for various personal reasons. The father tells them 'No', which they are shocked by, and also keeps it from the mother, who would have said yes in an instant. The mother is tired and worn down, but then returns to her acting career, after many years as a SAHM, becomes reinvigorated and revitalised.
By the end of the book, the DC sort out their various problems, the parents sell their large but dated house in London to downsize, and give money to the DC to help them buy their own homes. Everyone 'grows' on a personal level, the DC become independent, they develop an adult relationship with their parents who reconnect as a couple, life moves on, and I would say that there is definitely a happy ending.
Now, I have always been a big fan of Joanna Trollope (although I think her latest couple of books are not as good as the earlier ones). But when I read 'Second Honeymoon' for the first time, which was when my DC were little, I was shocked by the attitude of the father. I didn't 'get' it at all. How could he possibly turn away his own DC? I didn't like him or have any sympathy with him as a character. He was putting his own needs (returning to life as a couple rather than primarily as parents) and those of his wife (who would have wanted her DC back in a heartbeat, even if it meant drudgery and tiredness, because she loved them so much and wanted to help them). How could he do this to his own DC? Didn't he love them?
I then re-read the book a couple of years ago, shortly after my youngest had left for uni, and I saw it in a completely different light. Although I still wouldn't have responded as the father did in the book, I completely understood where he was coming from.
What we need to understand as parents is that, just as we are not defined by being parents, so our DC are not defined by being our DC. As the saying goes, probably misquoted, 'If you love a thing, you have to let it go...' To quote another cliche, you need to give them 'roots and wings'.
When your DC are babies, toddlers, of school age, you cannot imagine how they will be anything but your beloved DC, and that is completely natural. But when they grow, they need to detach from you, in order to grow and become independent functioning adults. I agree with other posters, including the OP, who say that DC regress when they come back to the family home. That is not healthy either for them or for the parents.
I know that many adult DC live at home with their DP and it can be made to work, if boundaries are in place and with give and take on both sides. But it is not an ideal situation; even if the DC is paying rent, doing their own washing and cooking, it is not being properly independent. And encouraging your DC to grow and become independent is what loving parents should be doing, not encouraging them to continue as dependants, which might be comforting to us (as we all want to feel needed), but is really not in their best long term interests.