Some of those guys can leverage their name for $$$ (like Phil Gaimon) but even then I don't think they can turn 6 figures a year out of it; unless they get hired for a job they're qualified for (which is possible). So you end up 10-20 years behind your peers with no real career, I don't think its too bad when you're in your 40's but once you are nearing the 60's I doubt many of them can retire comfortably.
One of my coworkers a few years back played in the 2nd tier of English soccer, and he had the same issue. That time gap is hard to overcome.
The really frustrating thing about being pro is the margins. The difference between a domestique and a GC rider could be that one puts out 6.1 and the other 6.05 w/kg. Renshaw was probably a top 20 sprinter worldwide, Gaimon a top 30 climber; but if you're not the best on that team you are bringing up bottles from the rear.