If people aren't running make with -j, do you think they really want their build to take advantage of all cores in the first place?
My argument is not specific to telemetry, it's a general one. If you have an option to do something that's not a default and it's not part of the software's core functionality, most people aren't going to consider it even if it would be to their advantage. For example, make -j.
It's for that reason that I don't think the "if they aren't turning it on then they don't want it" argument holds as much water as you think it does. That argument groups together three groups of people: people that know about the setting and don't want it on, people that don't know about the setting and don't want it on, and people that don't know about the setting and would be happy if it was on.
Ironically, if you had good telemetry you'd be able to figure out how many people fall in to each group and make decisions about settings based on accurate data. Without that, you're forced to work on assumptions.
> Your entire argument essentially boils down to, "I'm sure I can get away with taking my users' information by making it the default behavior with a buried notification and I can placate people who care about privacy with an opt-out."
I think you're making the assumption that telemetry has to violate your privacy in all kinds of heinous ways and therefore only be a bad thing. If that's your mindset, of course you're going to think that I'm the kind of person that's out to trick and fool people and betray their privacy. And in all fairness to you, it's reasonable to be jaded when companies like Microsoft have horrible things like P2P software update distribution enabled by default. It's reasonable to be jaded when you don't get told exactly what kind of data is being sent back as telemetry. There's a fine line between "this is good" and "you're just relying on people not knowing how to change the defaults in order to try and get away with horrible things" and all too often that line is crossed.
But I'm an idealist. I see the good things that can come out of having telemetry. I want to know if my software has started getting popular in locales that I haven't written translations for yet so I can commission a translation to improve the experience for people in that locale. I want to know if there's a setting many people turn off so that I can consider turning it off by default to match user expectations. I want to know if my users are sticking to older OS versions because if they are I need to keep older hardware around in order to test and provide them the best possible experience.
I don't think anyone would have an issue with software sending back that data (and only that data) if you clearly tell them that's happening, and I also think that most people would be perfectly fine with that being a default behaviour. Of course, there is always going to be a group of people that will have an issue with sending back that data and that's why I made the point about keeping it as an option.
That group isn't just "pesky users who I only care about for promotion" (perhaps I was too flippant about saying that in my original comment). They could be trying to harden a machine so that it only uses the network under known circumstances. At the same time, you'd hope that software intended for use by people who need comprehensive privacy like whistleblowers wouldn't have telemetry at all.
Given that OS X isn't 100% free software, no one should be using brew in that kind of comprehensive privacy situation and so having telemetry isn't inherently bad.
It's the "clearly tell them that's happening" bit that's the issue here. If you have software that has been around for a long time that doesn't do something and then in a new release it starts doing that thing, you need to let people know that! It doesn't matter whether that's telemetry or anything else - if it's something that violates existing expectations you need to tell your users loud and clear.