I don't think that holds up in a universe as diverse as that of Star Wars with all sorts of wild sentient beings. Particularly as most of the droids we've seen are of approximately human intelligence and surprisingly human mindsets. Whatever the secrets behind their construction, they're ending up seeming very human-like. So in terms of their intelligence and way of thinking they're more "peers" of humanity than some of the other aliens are!
I think that's a more important point. There probably is a dividing line. Where exactly that would be something for the droids to determine, probably. They'd have a better idea than humans would.
As
@billd91 says that's 100% been argued to be the case about humans, like repeatedly, for centuries. It doesn't really matter why something exists if it's inherently sentient, sapient, and free-willed. You also can really get into the weeds with that because some droids will not have been created to be slaves (c.f. C3PO), but are clearly treated as slaves anyway.