The reason that Heinlein's heroes are all supermen is because of the writer that most influenced his work. That seems obvious in retrospect, but few people seem willing to give credit to E.E. "Doc" Smith. His Lensman books were quite possibly the genesis for the golden age of science fiction. His Gray Lensman was the basis for practically all of Heinlein's male heroes. The guy was super smart, super strong, and super super. He was the perfect pulp hero. So were most of Heinlein's heroes, with the exception of the dirty old man parts.
The Lensmen were such a part of science fiction that they made a guest appearance in Number Of The Beast with no explanation or back-story, the reader was expected to be familiar with the characters. Heinlein tended to get a little philosophical about societal roles and their function in a culture. That is of course his privilege, he's the author after all and can put anything he wants in his books.
The way I figure it, if anything he claimed in his books was too radical they wouldn't have been so popular or won so many awards. Sure he could have coasted along on the success of his early career, but that wouldn't have worked for very long. The differences that most people seem to recognize between his old work and his new work is the difference between juvenile books and adult books.
A lot of his early work was done for pulp magazines and juvenile novels. The publishers wouldn't accept the racier stuff, but when he published adult novels the material was allowed. So his material didn't really change as he got older, the public just got more accepting. Even if was a dirty old man, big deal, at least it makes for good reading.