That is actually mozilla's stand in the linked issue except it's on client though. They would rather replace it with some non native replacement (So there is no surprising security issue anymore) if remove directly is impractical.

There is actually a example of such situation. Mozilla removed adobe pdf plugin support a long time ago and replaced it with pdf.js. It's still a slight performance regression for very giant pdf. But it is enough for most use case.

But the bottom line is "it's actually worth to do it because people are using it". They won't actively support a feature that little people use because they don't have the people to support it.