The option that allows you to pay extra to shorten the length of the game should not exist in the first place, even if the grind is justified or adds to the overall experience of the game. That is greedy and I don't know how else to look at it.
Let's be honest, we have had many cheats in the past to quickly play through titles.Then they started charging money to give players the ability to help them quickly finish the game. That in itself is something we can debate but purposely making the game feel like a grind, especially if it's not fun, and then having the ability to pay to skip the grind is not very ethical. HOWEVER, can the argument be made that many games in the past like RPG's also have a lot of grind built in them to pad the length of the game? So I guess that's ok because the temptation to get more money out of the player is now a non-factor and it's just a grind of a game.