There is a story about people getting driver's license having a check box to opt-in into being organ donors, and very few said yes. Once the box was changed to opt-out, very few said no :)
The question is are people saying no because they are privacy conscious, or because they don't care. My money is on latter. In general more people care about Firefox being fast than security.
What's a bigger issue for Firefox is deprecating its add-ons. That's going to hurt its marketshare way more than telemetry data.
On the other hand, everyone complains Firefox is slow.
So,few pay, few opt in, and everyone complains.