The end of XUL was necessary to make Firefox faster. Doenload an old XUL Firefox version and surf the web. If you don't notice a difference in page load speed, startup time, and overall responsiveness, you might have a very, very fast computer, which most people around the world don't have.

The 3 or 4 thousand power users that needed native interop were (and still are) of no interest compared to the then bad performance of Firefox in relation to Chrome. Even HN was full of people who complained about Firefox' performance. Now it's full of people who recommend to give Firefox another shot because it's fast again.

Firefox is also currently the only browser where uBlock Origin can use all of its features, Chrome is intentionally limiting APIs. Are people using Firefox because of it? No. So XUL add-ons would not have helped Firefox.

If you still want XUL, there are many forks out there that claim to support it.