189 seats at up-to-31-inch is possible in the old -800 cabin, it was actually Norwegian's original cabin layout. Constrained by the exit limitations* and payload-range performance on the -800, rather than seat thickness.
Thinner seats will give Ryanair 197 seats in the MAX-8 cabin, technically capable of 200, with two additional exits and no change of floor area.
* One of the reasons the -900 sold at all was that it had enough exits to permit the pitch to be reduced to 28 inches, so airlines had an extra bump in capacity beyond that suggested by the extra floor area.