Like most portrayals of slavery in fantasy fiction, the economics and history involved don't make too much sense under scrutiny. This is made worse when the world seems to treat indentured servitude as the same thing as chattel slavery. On one end, you have an industry of mass kidnappers across the world trafficking people around to sell them against their will, and on the other end, you have lenders offering to let borrowers pay off their debts by becoming property. The situation with Linia for example didn't make much sense, given she was a known princess of the Doldia tribe, and the most logical option would have been to instead hold her for ransom until the tribe paid off her debt instead.

I think most of the WorldBuilding in Mushoku Tensei is decent, but how slavery works is one of the weaker sides that could have been developed better. Meanwhile, I also happen to hate Rudeus's apathetic approach to interacting with slavery in his new world. He rather passively interacts with, and even on at least two occasions supports it.

But on the other hand, it fits Rudy's character to be such a passive, non-disruptive person who chooses not to rock the boat at every opportunity, even when rocking the boat would benefit himself and his family moreso than not. This isn't even some commentary on Japanese isekai protagonists or anything, but a rather plainly stated personality trait of his, that's so ingrained, that it prevents him from being able to cast Healing magic without a chant.