Here I am suggesting some minor changes to the referral system.
1) Have a maximum on the amount of SP you can gain from invited users. 250k sounds good and still very beneficial, especially since you are doing nothing.
2) After reaching cap SP you get 150 protocoins for every used you invite that buys premium.
or
3) You get 2 protocoins for every used you invite that reaches rank 2 and 3 more protocoins when they reach rank 5.
Benefits : Well suggestions one is to stop player from reaching super high ranks by just inviting user while others worked their butts of for it. Suggestion 2 and 3 is to encourage people to invite users even after they hit cap. Suggestion 3 has a long term benefit... if the player really plays to rank 2 then there is a higher chance they will stick with the game.