While key fragments might be theoretically infinite, the probability of getting a fragment does decrease with each one you get (resetting on a monthly basis), so actually getting 12 per month might be statistically improbable. (And who knows if it decreases down to zero if you play enough and get sufficiently lucky, we may well be capped at 12 fragments per month or something.)
It might be balanced right so we'll break about even, or it might be that if you ever take some time off you won't be able to manage 12 frags/month (and so end up with excess chests), or it might be the case that some will end up with excess.
Depending on the balance, it might be viable to allow keys to be bought with ip, just to save people some win grinding and as a possdible ip sink, but it might be too early to say.