That is a completely irrational argument.This is wrong. There is a difference between slavery and indentured servitude. If there wasn't, they wouldn't be different things with different names. Therefore it cannot be exactly the same.
Loads of things have different names but are the same thing, or functionally the same thing. In this case, they are functionally the same, and you've made absolutely no rational argument that they're not. You've just irrationally asserted they're not because their names are different.
This is also a terrible argument.Remove any of those things and the feel of the setting changes.
All settings change regularly. It's not change that matters, it's magnitude and type of change, and whether it actually impacts the main themes of the setting. Dark Sun is a great example of that - even back in 2E, the material added and changed in the second boxed setting significantly changed the tone of the setting and the hopefulness of it. Slavery is not one of the main themes of the setting - societal oppression is, but not slavery specifically - note that not all the Sorcerer-Kings are even very keen on slavery, nor is it a large feature of all the kingdoms. But they're all extremely oppressive.