People should be careful about this calculation. It's an interesting psychological trick, because your SP number increases doesn't mean that you have a greater value. The 9:1 rule doesn't mean that you earn 90% interest rate on SP, you should see it more like ~10% inflation on it. Regularly 1 Steem is generated out of thin air to reward contributors. The same time this effect is counter-measured by distributing 9 Steem to existing SP holders, weighted by their SP amounts. I.e. if you own 0,0001% of all SP, you get 0,0001% of the newly generated 9 Steem.

white paper page 38:

For the first 2 years of Bitcoin’s life the network sustained an annual inflation rate20 of over 100%. For the first 5 years it was over 30%, and for the first 8 years it was over 10%. According to the tool for estimating future inflation included with the Steem source code, Steem by contrast will achieve an instantaneous annual rate of approximately 12% after just 1 year (not including the effects of SMD operations).

This isn't necessary bad if more people buy-in and this could happen because Steem seems to be the first easy-to-use blockchain-based project with a viral component. If you compare it to Reddit and Facebook there is lot of potential to grow.