I see a lot of cross over chicks from some of the milf websites; but I agree, a lot of these chicks just do 1 or 2 scenes on a website and then disappear. Maybe they need a better pimp (I mean model agent.)
Or maybe they just needed rent for that month or books for college and answered one of those ads in the back of your local hip newspaper.
There is always a good amount of new talent making way to suck seed; the meat-machine is set up to pump out new sludge. As long as americas' suburbs keep growing with dumb chicks who learn to be nothing but cumholes by the age of 18, don't worry, we will always have new fresh chicks to wank off to. Some might cross over for that extra long 6 month 'real' porn career in the majors, others might just play some little league. As long as the roster is always filled, I say, who cares if some get flushed down the toilet of porn into obscurity?
I had an ex gf do a one time bj scene for doghouse.com. She never did another scene; which was good because she was a cum dodging chick who really just needed quick cash. I would assume this is the tale of most of these girls.