Muggleborns are quietly fed love potions targeted at purebloods and halfbloods, until they have their first pureblood child. By that point, the love potion has long-lasting effects on most of the muggleborns. For those who break free, they can get a divorce...but their spouse would basically get to keep everything, because a muggleborn is obviously less capable, etc. And the wizarding world hates divorce, so good luck finding or keeping a job in the wizarding world as the muggleborn instigator of a divorce.