I see this happen to novelists, too. Even really good ones seem to have a set of themes that ignite them and inspire them and they're dynamite. Then... they've said it. They got out what they needed to say, and they have the choice of rehashing the same old themes or going for new ideas, but ones that don't hold as much juice.
I think creativity is driven a great deal by a need to express oneself. Otherwise, it becomes a mere job or habit or challenge (such as Paul trying on different music styles). When you're young, you're on fire about a number of things. It's all new and exciting (or irritating) and you've got to get your two bits in. Then, if you become successful, you have a house and home and car and food. Where is the material? You're comfortable. I think comfort is one of the deadly enemies of art.
There is that awful ten years when rock stars are too old to be cool but still too young to be nostalgic and they should be frozen some where.
This is so true! Listening to Ringo's work, I really love his nostalgic stuff. When he sang "Weight of the World", I could picture him telling this to John. The older, wiser Ringo trying to share his wisdom with a friend. We get that in "Fade In Fade Out" and also some of Paul's MAF. I think George's "Brainwashed" definitely falls into this category, and his Cloud 9 falls into the previous-- coming back to something rejuvenated after a rest.