How valuable can simple circuit training exercises be to maintaining a decent body

Something is generally always better than nothing but I agree with @sacbuntchris and @HappycamperNZ that progressive overload and changes in how many muscles you target throughout the week are necessary for progress if you want to either build muscle or improve overall physical capabilities.

While our medical records are all different, what you're doing is probably beneficial for overall health -- cardiovascular stimulation, warding off bone degradation, maintaining muscle, etc. Even a basic routine and nutritional intake just maintaining muscle without building it is better than letting it degrade while doing nothing.

However, this also really depends on what your current circuit actually is. Are you doing three sets with five pullups, five pushups, five crunches, etc., per set? Or are you doing three sets of 15 pullups, 40 pushups, 80 crunches, etc., per set? Are you doing high reps per set everyday so your muscles never have a chance to fully rebuild? Are you doing low reps per set or only doing your circuit once a week? Does the circuit take ten minutes and end before you can get enough from it? Does a circuit take over an hour to where you run out of energy and your body starts destroying itself in a bad way to try and make up for the lack of fuel? These kinds of things make a huge difference too.

If you don't care about increasing strength, building muscle or losing fat and just want to stimulate your body for a healthy heart, stress relief, etc., then what you are doing is probably fine depending on how many reps and how often you train, etc. But, it sounds like you are wondering about reaching a plateau and looking to progress further, at which point anything from higher reps, added weight, and different targeted exercises, to focused muscle group days and a planned recovery schedule will give you multiple ways to incorporate changes and progressive overload effectively.