Moreover, at present, we have no idea how many people use each solver (and on which platform!). Knowing how many people installed which solver
   would allow us to prioritize support from our finite developer time.

Why not just let users vote on that? The support is for the users, no? Instead the developers want to minimise the amount of time they spend on maintenance based on the number of users who could potentially complain. The reason for this is (as we are about to be told) so they can spend more time working on platforms where they believe commercial solver developers could provide for-profit support services "(or $$)".

   This would also allow us to lobby the commercial solver developers to provide official support (or $$). To quote one company "We'll want to
   provide official support at some point, but it looks like the scales haven't tilted quite yet." It'd be nice to know whether 100, 1000, 10000, or
   100000 people per month use their software; that might change their mind.

The truth comes out. Collecting data via "frictionless" telemetry allows someone else, e.g., commercial solver developers, to make money. Nothing wrong with that if we let users know about these intentions, however when devlopers try to operate under the guise of "free", "non-profit", "open source", etc. while, truthfully, they have commercial motives, then it seems to me they are doing everything they can to avoid tipping users off that this aims to be a commercially-oriented project. Instead of just being transparent about their motives and letting users decide, they want to sneak something by (most) users. The issue raised here is not the collecting statistics (nothing wrong with that), it is the less transparent, opt-out nature of it: telemetry. Deceptiveness, stealth. The message coming from this discussion is "Don't tip (majority of) users off that we are collecting data." And why is that? Because the developers know this is something most users do not want.

   Finally, if it is opt-in, the vast majority of users will not opt-in. This leaves us no better off than we were before. Opt-out is a good
   compromise.

The discussion should have ended right here. If providing usage statistics is something that the Julia developers already know the vast majority of users do not want to do, then sneaking it by them via opt-out telemetry is wrong, and it tells us much about the people behind Julia. If users do not want it, and you know that, then why the heck are you doing it anyway? Anyone reading this will know why, but most users will probably never read what we are reading here.

The rest of this discussion devolves into "Everyone else is doing it". The lone dissenter finally gives in to peer pressure.

I remember when using download statistics was enough. Developers still maintained software. No "trade-offs" were needed.