The thing you have to remember is that the stamina invested into training the ability sets you behind, and you're not seeing a return on your investment until you "pay it back". It's possible to spend, what, 40 stamina training cardio in the current update? And still no upgrade. Even if the upgrade was at 40 stamina, it would then take 40 days before you "paid it back", meaning you're not seeing a return on that investment until halfway through the third month. And you're probably continuing to invest more into the next level, meaning you're only increasing your "debt". Meanwhile, all the stuff you could have gotten instead are going unused.
It's true that things can inflate out of control if it's too easy, but I've also played games where the highest upgrade of "increase the gold dropped by enemies" never pays back its cost until long after you've gotten enough money to buy every other upgrade. It's extremely difficult to balance, and even if you do, it's frustrating for the player to try to figure out whether the dev of this particular game balanced it properly and if it's worth the investment. After putting a lot of thought into this issue, I came up with the following general rule: Never have a feature that allows you to "invest in your ability to invest" with the same currency as everything else. Make something else upgrade that feature. In classic arcade-style games where enemies drop gold for upgrades, I'd recommend having gold drops increase the more achievements you complete. In a game like this... I dunno. Maybe have stamina upgrade with each client whose therapy you complete, or when you have sex with a certain number of different women, or something?