If it is however not fun and those running it are adversarial to the users then it does not work. I posted a problem here that is reasonable.
Well no, you posted incorrect assumptions about DCs (that they offer alternatives, and that negotiations require diamonds). Now you're leaning so far into victimhood you're accusing the company that runs the game of being adversarial to players. That last is especially unreasonable.
The game, maybe in the Story line, pushes for an Age upgrade that makes it impossible to continue with the event.
I'll repeat: that depends on the state of your city. You have placed your city into that situation, Inno didn't. The game is about how efficiently you use your resources, and one of those resources is time. The game is about choices and trade-offs. Choices have consequences, some of which don't become apparent immediately. Events tend to reveal shortcomings in game play that one doesn't see in normal day-to-day play. One can either plan for them, or not. That's another choice.
INNO should have made the Age demanded of the units or negotiation to satisfy the quest dependent on the player's capacity or built in some other work around, like a payment not in diamonds.
The negotiations don't require diamonds in the first place. If you don't have enough goods to repeat the negotiations, that's not Inno's fault. If you have advanced an Age without proper planning, that's not Inno's fault. If you have already advanced as far as you can on the Campaign Map, that is not Inno's fault.
The response I got was simply abusive.
There's a difference between blunt and abusive. When you blame Inno for your game play, and ignore the advice given to you, that's a lot closer to abusive than anybody has been on this thread.
You are not a victim. Inno is not out to get you. Change your perspective, adapt your game play, and the game will be fun again. Cling to victimhood and nothing will ever be fun. The good news is: that is entirely within your power. You don't have to sit around and wait for Inno to change anything, you can fix it yourself.