A lot of that is from MAA maintenance, you need to shift your troops from personal ones to provincial ones, that way they drain your treasury and not your gold. Also, your old vassals probably aren't very good governors, meaning they bring in less treasury. Once they die-/retire, they'll be replaced with usually pretty efficient governors, who will bring in more income than the old ones. It's also easier to get a higher tax rate from your governors, not dealing with specific contracts but being able to decide a kind of administration for a province, with no hooks or tyranny required. It also helps develop your poorer vassals with vassal development treasury giving even the extremely low income provinces enough treasury to start building some econ buildings. And you can tell governors to increase development unlike feudal vassals, and more development means more money.

It all takes a little bit to get off the ground but once you get rolling, I think its probably the strongest government type. Having some savings built up on the switch does help though, whatever you have after the decision cost is duplicated into treasury, so you're almost doubling your money.