So every day less new bitcoins mined? This is nice.
I think it is the same amount every day until every four years, at which point half the previous amount is mined every day for the next four years and then it repeats.... until the year 2141.
The process you describe is correct, if you plot it as in OP, it shows an asymptotic convergence on 21 million (20999999.99999999 around the year 2141) when the last satoshi will be mined.
It is due to the underlying geometric series, ratio 1/2,
10.5 million * (1 + 1/2 + 1/4 + 1/8 + 1/16 + ...) = 10.5M * 2 = 21M,
where the 10.5 million comes from the first 210,000 blocks @ 50 btc per block and the ratio two is the halving
Bernoulli spiral.http://www.matematicasvisuales.com/english/html/analysis/seriegeom/seriegeom2.html
If by the 2140s we need more satoshis then the significant digits to the right of the decimal place can be extended in the protocol without affecting the 21M btc limit and mining for btc will continue.