Given that it's known that power users disable telemetry, developers should not rely on telemetry alone when deciding to remove a feature. The opposite is spiting power users, and you can see how that's going for Mozilla.
What better data should they use?
> you can see how that's going for Mozilla
HN commenters (not necesarily readers) react this way to most changes in everything. Mozilla doesn't stand out IME.
For one, they could take comments on their bug tracker to heart instead of locking the discussion as soon as there are enough users unhappy with a change.