We should gather statistics about the number of successful-login notifications people would be receiving if enabled globally.
There will be a number of different "groups", but if a number active users (ie. for which we have a bunch of data) would be receiving many succesful-login notifications (we can assume that all of them would be False Positives), I don't think it would be suitable for enabling by default.
Some data might be on the Notifications database, although that's not clear, and we probably need some extra code.
I would eg. run it for a month storing the number of succesful-login notification attempts that would be sent per user, then show a table with the total number of logins, and the number of users that would have received N notifications (perhaps shown in ranges, for N > M).