Semantic drift is a real problem. Too much semantic drift can make it difficult to communicate, because we no longer have a common vocabulary to communicate with. I don't think semantic drift is inevitable, but it is inevitable if it isn't resisted by a large enough fraction of the community.

The whole discussion here is about semantics. What does "open source" mean? I think having a clear definition of "open source" (at least when it comes to software) is a good thing for the world and for the community. It helps us communicate to have the OSD definition. If "open source" changes its definition to no longer require no restrictions on usage, we have lost a good and useful term that will make it harder for the "real" open source community to communicate.

By analogy, imagine if some people started arguing that ceasar salads (with chicken) are vegetarian, because salads contain mostly vegetables. Wouldn't it be reasonable for everyone to resist that semantic drift? The term "vegetarian" would lose both its history and its usefulness.

Those who state historically accepted definitions in strong terms are trying to resist semantic drift. It doesn't mean "they aren't open to being convinced of anything", that's a bit uncharitable. It means they aren't open to semantic draft regarding this term.