1) When still alot new users come
2) When rich users stop playing (and can't give away their money anymore)
3) When someone looses money by buying other money from the bank (there's a little loss in the calculation included)
4) When bones got too old and disappear
5) When server admin disables ore generation or does something else to decrease money
6) When players almost can't reach the beginning of the generation position
7) When rich people have everything they need (= no buying of new stuff)
I think there are alot factors to stop the inflation

EDIT: You should take it easier...Minetest is not the "world", there aren't much other factors which could make an inflation.