If I may!
Your suggestion sounds good in theory but unfortunately that’s where it ends up staying “a theory” as it can’t be put into practice because it’s doesn’t suit this business model.
Marketing strategies are very carefully analysed with months of discussion before they are implemented using data based on what, where and when each player spends the most money on. They collect all of that right down to the last click you do m8.
The current iffers make you buy them mostly for the gems offers really knowing that your most likely to use those gems there and then on summons thus they are not about the mats at all. Those added extra items are only included to make it seem more interesting for you to purchase. Sorta like a chocolate bar on special on the counter at the cash register, it’s tempting and in your face.
This is the same sorta of concept, the mats are tempting and in your face.
Now if they gave you an option to choose what you wanted as you say once a month then they would sell gems anymore because everyone would be waiting for that offer every month and thus affect every other offer that doesn’t have an option.
Because you see once they give in once to this kind of offer, based on human nature,The next complaint and/or request would then be “Why can’t all offers have this option” and then the cycle of theories starts all over again, it becomes a never ending story.
Your idea is ok for the odd few but when your talking 100’s and 1000’s it’s a whole different ball game.