imo: there is another problem, when you need double you power you need more and more food (10 40 90 160 250) and time or salt.
10->40 is ok when food provides 1-10 FEP (~ 30*3=90 food items => 10 per day => 9days),
but when you wanna 160->250 and food still gives 10-20 FEP... (90*10=900 crafted food and you still can eat 10 per day => 3month for same boost as at first 9 days) //here hunger becomes huge problem if you wanna dig dipper and faster
so the longer the game, the longer it will take to maintain the speed of development (and players mostly leave after few month)
and without x30 FEP boost (subscribe, drinks, hunger, symbel, pepper, truffle) game becomes tedium shit, where you gain 1 stat per day and it means less and less, every day