I’m from a gera family so I know from my brothers about the 10-15% “survival rate”. This is talking about the gera in America and not in EY because I wouldn’t know much about there in this sense. If you keep your eyes and ears open you could pretty much tell, though obviously you can’t know for sure. Also, it’s possible that a larger percentage do keep some of the chumras but not to the extent that everyone talks about – (which is only for the first year or so)
To 2scents: When a gerra boy marries a non-gerra girl, it really depends if we’re talking about a boy who is more open-minded and therefore meeting a girl who’s not ger or if its the girl who doesn’t mind living with the gerra derech and therefore meeting a gerra boy (not too common). In this case, because it’s the boy who’s open-minded, he probably knows more than other bachurim his age and therefore he’s not even ready to live with the gerra chumros himself (these chumros are much easier to accept by those who are temimusdig or up to that level?). Therefore he probably WANTS to marry someone that’s not ger while still remaining in the Gerra Kehillah since those that plan on keeping to the gerra chumros will only be looking for gerra girls – since it’s an accepted rule that only those that are from a gerra home/gerra chinuch are ready to live with the strict chumros.