Just because it was planned 2 years in advance doesn't make it any less meaningful. And there were a lot of "incredibly specific workflows" that xul extensions allowed that webextensions won't. Will you deny millions of people each their own incredibly specific workflow if there's one singular ecosystem that mozilla has to maintain, to accommodate them all?
I would say yes, since web browsers should prioritise performance and security of their primary use case which is accessing the web. In my opinion extensions are just a nice add-on, not the reason browsers exist in the first place. Most developers understand from their own experience that backwards compatibility while admirable can be very costly to maintain forever.
