Well, I'm not so sure if Steem needs rewards for creating an account. It is actually growing quite rapidly when you take the number of accounts. Steem is approaching now about a million user accounts.
Also, Reddit got 200+ Million of accounts without paying referral fees.
I see other issues that are limiting the growth at the moment.

• Account creation takes forever! when I joined October 17 it was 5-7 days and now I know from other users that it is often more than 14 days. There is no need to pay for referrals if there is no technic to validate them in a timely manner.

• Consuming Information on all the current platforms is to difficult. Having only Keywords to structure the information is technology from the year 2000. Reddit e.g. has Subcommunities, Facebook and Twitter have a personalized feed. To keep growing the Steem Frontends need to develop further to make it a lot easier for a user to consume information not just write them.