It doesn't have to make sense to you though, it makes sense to Supercell. They have done the analysis on their end and have determined that the way that gemming upgrades works today is the way that makes them more money.
Think about it......... if they could make more money by offering upgrade boosts, then why wouldn't they? It must be because they make more money off of players who are so addicted to this game and have enough money to gem upgrades to finish IMMEDIATELY, instead of giving people the option to spend fewer gems to speed up a long upgrade.
They have a lot of smart people working at Supercell who have made a very good living by understanding how to determine what makes their company more money. You must agree with this.
If they haven't changed the way people can spend gems on upgrades, it MUST be because they know they'll lose revenue if they did that.



