In order for a trade-off to (possibly) be good, users need to know that they are making a trade-off in the first place.
How are users going to be informed of the trade-off and the mechanisms to opt-out?
I ask because I follow the Julia issue tracker and read discourse almost every day, and this is the first time about I hear about telemetry in Pkg.
An additional thought: how are you going to use install numbers to estimate usage numbers?