I was listening to a CBC radio segment about them the other day, and one of the producers was interviewed while she was at Stadium Station. She asked a few people if they'd get one, and a large chunk said they'd wait and see how the launch went first. I don't think a slower uptake is necessarily a bad thing for now.
As peoples' December passes get close to expiring, they see others using the cards, tickets, and vending machines, and they see that it's been working so far, I think a lot will make the jump since they can be confident that the convenience won't come at a cost of dealing with a lot of bugs and outages. If uptake is still slow in the new year when they're rolling out the other fare groups, then I think they should consider ending passes and paper tickets early (they're already planning to phase those out at some point anyway).
My concern is that if adult fare payers are forced to use it so soon, especially when other fare groups aren't able to use Arc yet, it could create a lot of unnecessary ill-will towards Arc—including among people who could have decided on their own to switch over if given a little more time.
Where did you hear about people disinterested in free cards by the way? I've been seeing a lot of questions about where to find them, I'd love to read more about how that's actually going.