The comparison of prices is not quite correct. People compare the current street price of the S975 ($1,750) to the RRP of the SX900. One should compare the RRP's of both at their introduction date, because in a year or so, the SX900 will decline, as you suggest correctly. The PSR-S975 had an RRP of $2,599 in Februari 2018. The SX900 has an RRP of $2,799. That's a difference of 7.7%. Not 40%.
If we extrapolate the price graph of the S975 (-32.7% in 18 months) to the SX900, it would be possible that the SX900 will reach a price of $1850-1900 in 2020 Q4.