Otherwise you'd have tons of extensions that could be rewritten as WebExtensions but aren't, because it's less work in the short-term to just fix them for the XUL changes. In the long-term, however, it would be a lot more work, especially if Mozilla starts recklessly breaking things.

Similarly, many unmaintained extensions will disappear and WebExtensions will be written to replace them. Those will almost certainly have better quality and performance. Shouldn't be all too hard to pull off for an unmaintained extension.

And then there's also stories like with NoScript's transition, which very much is a lot of work, but it will leave the NoScript maintainer with a much cleaner code base, not just from the WebExtension API being cleaner, but also simply because he rearchitectured the whole thing, as he was forced to rewrite anyways. And he'll get significantly better performance out of this, too.

He would not have done a rewrite without this, but he's glad that he did it.