First off, let me say that I really enjoy the game , but I think this new business model of restricting dust and diamonds in game, basically forcing people to pay money to upgrade their creatures, it's a mistake on your part.
I understand that you want to slow the progression, but I've been playing daily for 6 months and spent around 300 USD in that time, and still haven't maxed out any creatures. My highest being 3 rank 6.2 epics with 4 or 5 special ability levels to upgrade before they're maxed, so I don't think there's much risk of people "finishing the game too quickly". Plus there are over 40 creatures in total!
This new restriction on dust and diamonds in game effectively makes it so that you have to spend money to progress. It's not "free to play" anymore when you can't effectively strengthen your creatures special abilities without spending money, which, let's be honest, is where they really gain their strength from, and it's what makes each one unique.
I used to look forward to new creatures being added to the game, but now I don't even bother with them because of the cost to upgrade them. Releasing a new legendary should have people excited, but it seems kind of pointless to me because I'll never be able to afford the upgrades necessary to make it viable in battles.
I was ok with spending money when I felt like I was getting a decent value for it, but now it seems like the idea is to get people to spend out of frustration, which takes the joy out of the game, and I feel that it'll hurt your business in the long run.
In my opinion, the progression rate was fine before. If you want to make more money, have more special offers in the shop that people will feel good about spending money on. $5 to $15 chest that have some real in-game value as far as upgrading your creatures go.
As it stands now, I think this update will hurt the longevity of the game, and I hope you'll reconsider your approach.