Currently, the referral system assumes infinite growth in player numbers is possible. Each new player is assumed to be able to find multiple other new players, in order to get the rewards. Even in a literal best case scenario where every person on earth joins, you are eventually going to hit a max where there's no one left to recruit. New players are essentially a limited resource.
I think it would be beneficial to the longevity of the system if you allowed players to also take a quality route to referrals instead of just quantity. It doesn't completely eliminate the resource exhaustion issue, but it will slow it down. Plus, players who will stick around and play the game long term are very important! So I suggest letting players get additional referral points when the people they refer reach additional play milestones.
Some possibilities:
- Reaching even higher companion levels
- Reaching a certain profession level
- Unlocking a certain number of den spaces
- Having a certain number of high level wolves
- Reaching certain points in the campaign
tl;dr- allow players to get all the referral companions by potentially referring one very long term dedicated player.