What do Seiyuu do after they are no longer at the peak of their fame?
What I'm asking here is what most seiyuu do after they are no longer
  famous


Firstly, the assumption that other professions with an early retire age generally have a following job that most workers transition to is not entirely accurate. For your NFL example, according to this, only 19% of coaches were former players and when you consider the much faster player turnover rate and the fact that there is far more players than coaches, it is clear that most players do not become coaches.

In the respect of what seiyuu do after their peak, they are just like other professions of a similar nature. If someone needs money, they try to get the 'best' job they can with their skillset and connections. What someone's connections are, and what they view 'best' to mean is so subjective, and varies so much from person to person that it is impossible to point to a specific job as what most seiyuu do after fame. (and thats before bringing in the ever-changing job market)

The next job is still likely to be at least partly related to seiyuu work, since that is their skillset and they probably made some connections along their career that could offer them some other work. However, this is not necessarily true and because of the variation of job market and each person's circumstance, it seems meaningless to point to any specific examples of working ex-seiyuu's... Someone could probably make a very long list, but hopefully this should answer your question to satisfaction.