There are plenty of avenues to spend MIL on
if you bought CS, and can thus spend it on provincial manpower upgrades (like you used to be able to
without buying DLC)... but they've at least mildly sabotaged that, because increasing your provincial manpower increases your development (which from 1.16.2 will be the basis for the cost of corruption reduction) without increasing your income.
Not entirely true. Each point of development adds .20 base trade power to the province. How this scales compared to the corruption cost increase depends on a number of additional factors but you are most likely correct in it being a net negative from a purely economic standpoint. Of course that ignores the other global development province effects as well as the military specific development effects.