I am fully aware of the convert operation availability. However, the majority of people that had SBD did not take it upon themselves to figure out how to use the advanced wallet since they barely understood their wallet. Most people only know how of the Steemit.com interface to their wallet. The next best option for them was to use the internal market. This was something that was intentional by witnesses and Steemit Inc. They intended for people to take the option of using the internal market instead of using the conversion option so the supply of SBD would stay high.

You are mistaken about the STEEM inflation. It is currently not following the inflation rate as you described. This is why there is over 11 million new STEEM created during the last 3 months on top of the normal STEEM inflation.

The inflation rate you described would appear like a straight line. Just look at this chart. This is the actual data taken ever single day since the middle of February of 2018. By the way, the inflation rate you described is only an approximation. To be exact, it started at 9.5% and is reduced by 0.01% every 250,000 blocks. Current inflation is 8.59%. Now you are equipped with the current inflation rate, go out and collect the data yourself and do the math. You will be shocked.