The reasoning for not making this opt-in are in the blog post and I find them reasonable.
Opt-Out is simply not acceptable.
In general, when looking at telemetry data, you can't stop at asking "what possible use would the org collecting it have for it right now, in their current business model". You need to also ask what use they could potentially have for it in the future, especially combined with other data they have access to. A year from now, or five years from now, or after a pivot, or after partnering with (or buying) a company in a different industry, or after leaking (or selling) it to third parties.
When you ask these questions, I think you'll also conclude that enabling telemetry by default and pinky swears they're not doing anything naughy with it is not good enough. You may trust the people doing it now, but as experience shows, business priorities change, people in charge change, but the collected data remains.
There's many ways to get valuable data for ad targeting, this is not one of them.
If they want to turn bad, they can just use the dependency proxy.golang.org / sum.golang.org that already exist for a long time. Or gather data on things running in Google Cloud. All of these would be unlikely, and bigger levers than gathering some data from a few Go developers.
The pushback here may seem paranoid for the specifics of this particular case, but that's because it comes from a more generic and IMHO perfectly justified position of treating all telemetry as risky by default, and all attempts at making it opt-out as wrong, dangerous, and likely malicious.
The arguments "for" are all based on assertions about this particular case at this particular point in time. The data to be collected is benign. Collecting it will translate to meaningful improvements being delivered faster. The people behind the project are all nice and reputable. And so on. In contrast, the arguments "against" cover the possible future evolution of the telemetry itself, as well as the project and the people behind it. The set of metrics and reporting frequency can change. The goals behind it can change. The nice people might turn naughty, or be replaced by scoundrels.
The arguments "against" include, in particular, building the precedent and infrastructure. From that point of view, you saying:
> set of static, anonymous metrics, updated _once_ a year from a tiny fraction of developers is not worth the effort to even ingest into some targeting system
sounds to me like me saying "this browser extension, which code I audited myself, is only reading the account numbers and names from my banking page - it's really not worth anyone's effort to do anything naughty with it, so there's no reason I should be afraid of giving it full read and write permissions for the banking page, and no reason I shouldn't leave the 'auto-update' checkbox on".
It's not about how interesting the data is to them, it's about the hubris of believing they are entitled to every last bit of our data.