Obviously I don't have access to the wealth of data Leaf does but I still feel fairly comfortable with saying that the majority of Itch usage will not be people in competition with each other. You only have to look at the excess of jam projects and experiments that reside here.
It's very important that Itch stays a place that welcomes these sort of projects as they're fundamental to our progress in games - freeware and related always have been. It's also important that not everything be, as you call it, doggy dog because *not everything is*
No-one in this thread is averse to the availability of analytics but I think putting the onus on Itch to consider its role in the community and how much "business-ifying" everything absolutely will effect the role it plays (because it always, always does) is vital. Part of the reason Itch thrives in opposition or accompaniment to Valve's sell all the things, everyone fight now is because it is, for many people, in opposition.
I understand that in order to survive and grow, Itch will have a balancing act of pleasing two fundamentally opposed camps - especially if it wants to invite more projects from professional game makers. Just listening to those on the other side of the fence from that is absolutely vital too - they don't want or need to be placed in competition, if they did then Steam already exists.
Personally, I would go with an opt in but I understand that the more friction, the more difficulty in getting some uptake. It's not an easy problem to solve but it is vital that some sort of compromise scenario is reached because what Itch does now is too important to throw away.