As others have expressed here the reason few people opt in to data collection may be because they have chosen to use a Web browser that does not mandate the collection of data.
I'm assuming there will always be an opt out which I shall add to my list of things I have to do when installing Firefox.
On Linux it may be that the various distributions decide to repackage Firefox with the default setting flipped. Not sure about the various policies on that one.
The ESR track presumably will have the default flipped because corporations get funny about data transfers to remote servers - mind you Microsoft seem to be getting away with it for business who don't have a full on Enterprise set up.
The way I see it is that if Firefox's userbase dwindles because of this, either we get our Firefox with opt-out telemetry or... Firefox dies. And now we have a Chrome monopoly.
Then don't make the compromise.
As others have expressed here the reason few people opt in to data collection may be because they have chosen to use a Web browser that does not mandate the collection of data.
I'm assuming there will always be an opt out which I shall add to my list of things I have to do when installing Firefox.