This is exactly the issue with telemetry, it's a vicious cycle. Most power users will turn it off because it's "spyware", so the only telemetry they get is from average users who don't even know the feature exists, which leads to them removing useful features, so in its current state it's practically useless for actual measurements and only ends up hurting things.
That's why I don't like when there's a blind hate mob against telemetry, because it can be legitimately useful for developers. People should focus on voicing actual privacy concerns and trying to get them addressed rather than completely rejecting any form of telemetry no matter how anonymous they try to make it.
But on the other hand, Mozilla should be well aware that a large majority of their user base probably turns off telemetry (I'm not sure if there's any telemetry sent back that says "user turned off telemetry", maybe not because some people are outraged even by that, but regardless they should know that) and not arbitrarily remove features that are "unused" when their sample is not representative. I highly doubt a feature like this causes a huge maintainance burden that necessitates removing it.
I'm not entirely sure what kind of data Mozilla's telemetry collects, so I'm just more talking in general and not claiming that it's fully anonymous and there's no reason to be concerned at all. You should definitely be allowed to opt out (or maybe even make it opt-in - that does reduce the sample size even further but maybe if they showed trustworthiness then more people would be inclined to opt in), but IMO we need to find some solution that benefits both sides.
The only other alternative I see is having some sort of forum where they make discussions/polls to ask users about how much they use certain features. But that requires active user effort and I imagine a lot of people couldn't be bothered to do it. Shoving surveys in users' faces would just annoy them, but not advertising it would result in a lot of people not knowing about it and missing out on an opportunity to vouch for their favorite feature because they weren't part of the discussion.