The idea is good. The money can come from the income of the referred users, in a way that it not hurts the system and the referred users. Because os this, the amount must be very low as 0.1% to 1%.
And, only Steem Dollars can enter in a referral system and must only generate revenue for the referrer if the referral is active and produces content that generates the income.
Any other way I do not believe it would be possible or interesting. Refer a lot of people only to say "Ohhh! Steemit has 100 million users!" has no value for the system. The value must be generated by the content.
My 2 cents...
Exactly! That's why I mentioned having a restriction on when you can earn rewards. For example, a post has to reach a certain number of Steem Dollars earned before the person that referred them is eligible for rewards - to boost quality and not just quantity!