It is impossible to say what the final number of coins will be.  Dash has a flexible reward system.  If mining wanes because it's unprofitable, the rewards go up according the the formula posted on the OP.  The Budget system could lower that, but a lot of the estimate is based on the lowest rewards possible and a possible "high maximum" which is a complete guess.

The reason for this system was to be sure that if interest in mining waned, the reward would go up and make it interesting to miners again before hash drops so much as to make the system vulnerable.  Everything in Dash is kind of on a checks and balance system, it's flexible.  That's why it's always been hard to answer the question "how many coins will be mined?"  To get more of a handle on that, Evan implemented the 7% drop in rewards every year (including minimum rewards) So now we at least have a probable range of final number of coins around 150 years from now.  It was also a minimal change to the system as far as the near term user's experience.

Even so, as long as Dash is around, it'll probably be making rewards 1000 years from now, at the duff or micro duff level, thus still creating dash, so technically, we still can't say how many

Originally, Evan had no estimates on total number of coins, but the community kept asking.  So he calculated 100 years out, and estimated an average hash rate that he thought would be good and wrote a "maximum" 81 or so million.  Then people started realizing it was not calculable and demanded he change it so there was a limit.  So Evan created the 7% drop each year, as it was minimally invasive and that "calculated" out for Evan at 22 million coins.  I argued that it would probably hug the minimal reward and be more like 16 million.  They felt that the hash rate would probably not stay so strong.  I think my predictions have been more accurate   But in the end, there is and can not be that accurate an answer so there is no point to changing the original estimate.  You'll have to wait 150 years to see for yourself.