My wife and I have been married 40 years with no kids or, as I tell people who ask, "We stopped at one kid -- me."
We go to WDW every year and have done that for the past 20 years. With any luck, we'll be doing it for the next 20 as well.
Why do we keep going back every year? Like most things it's not for any one reason but one of the main reasons is that this is a place I can truly relax and just be myself. I have a high stress job and the days we spend at WDW allow me to unwind in a way I can't do any place else. Also, there is just so much to do, different things to eat, shows to see, rides to experience, shops to visit, tours to enjoy, etc., etc., that each year is different.
We don't get any "hate" because of our annual visits but we certainly have gotten plenty of odd looks which I happily acknowledge. I will say that I have had instances where the same individual who initially gave me the "odd look" came to me for advice regarding their own forthcoming trip to WDW.
As others have said, WDW without kids means you can get up early (or sleep late), go to the parks (or not), have a shopping or pool day (or not): in brief, you can keep your own schedule.
Finally, we have taken our nieces to WDW twice when they were children and had a great time. We have also gone with adult friends and had a great time.
It's all good.