Your arguments involve a lot of assumptions. For example, that the heirs of the other local forces are any better. Or that mercenaries have the foresight and free time to worry about their future. Or that being a medieval mercenary is considered a stable job where people care about their employer and their retirement. Or that job opportunities are plentiful and mercenaries can change employers at will.
If I were a member of a ragtag medieval private military force and a child of the lord came and asked for a woman with big tits, my first thought would be, "Finally, a lord who speaks our language". Watching rugged warriors act like prim ladies amazes me. Even assuming that a child shouldn't act like that, the fact that the parents don't pay any attention to him is a great opportunity to mold the rich and soon to be influential child into whatever you want and reap the benefits. It's never too late to leave.
He's considered the worst character in the game, so, yeah, any heir from any other family by definition is better.
These are feudal times, one of the things a lord must have and maintain is a force to keep the peace of the land he controls or when the king asks, to contribute to the war effort. I'm not saying it's stable, but it is in demand. And the mercs obviously care for who their employer is because the maid stated that all the good ones left after having had contact with the heir. This also speaks to the job opportunities being plentiful and that they can change employers at will since none of them came back to ask for their job back.
As for these "rugged warriors act[ing] like prim ladies", well, that's how the mangaka depicted them. In any case, the mercs are not going to get a chance to mold him, remember he's leaving for the academy soon.