There's something wrong with the numbers as you get higher in level. The steam forums bring it up enough times. The developer tried explaining it away just like you're trying to do, but it's a problem with the numbers. Basically, the values on the career tree for -happiness and -health are being multiplied by the rank or something. It creates ridiculous -health and -happiness values near the top where the salary isn't really enough to keep you happy with all the bought items.
The rank 6 job had me at -137 happiness per day.
Considering that a major part of the game is happiness and health management, this is a SERIOUS bug.