The game has a few areas that need hard capping, once you start really upgrading the numbers start climbing to a larger and large degree, never quite going exponential but not far from it, i was able to start pulling in 100k coins and gems each dungeon run and the rarer materials also started to grow hard and fast. It made me nervous, i don't want to fuck my save by continuing to play, if things keep going at the rate they've been going i wouldn't be surprised to see trillions and coins and crystals pouring in and upgrading the girls to ultra-god power levels.
Basically, hard caps, some upgrades are capped, others not, even if you want to increase the caps later i still think there should be sensical caps at this point, something like a max of ten core upgrades for each maid and the MC, same for the iron and platinum forge generic upgrade and similar in the other categories.