If you aren't going to change the "Open ______ chests in the Tower ____ times" to something else, then you need to make it so we can destroy Common (or whichever) chests to make space. That way, at least we could grind for it.
Even if you bought all the chest slots, you could conceivable fill them all with chests other than the one(s) required for the mission.
Doubling the prices was a mistake. Jacking up the cost of refresh by 10 times was an epic fail. You don't understand how Gacha games work. Very few people are going to drop $100 on an in-game purchase. The idea is to nickel and dime people to death with $1, $2, and $5 buys, and you have to make it worthwhile for people to spend it. That's how you make your money.