It is my opinion that Siwa would be better with a tiered spender and here is why.
Money, Airea is a for profit company and if the game does not make enough money it will be shut down. Tiered spender works or it would not be in all their other games.
It makes game more attractive I like to get the most bang for my buck. Lack of a tiered spender is why I went back to Sanctum and so did several others.
Better economy Law of supply and demand means more supply equals cheaper prices. People sell their unwanted tiered spender items in AH. And with 50 M-clay for spending just 399 AP it means more stuff from the clay tables. (personally I think anyone who plays GF long term should do the recurring AP even if just $5. Then you can do the tables to get stuff to sell and have the gold to buy what you need.} An example is the lacy maid outfit which at 1 point went for as little as 1500 gold on Sanctum, while on Siwa it was 4000 or more. So having a tiered spender actually helps free players by lowering prices.
So if you want Siwa to survive and players to have a better experince then give them a tiered spender.