Eventually the diamond buyers will get fed up and stop. I would of given up already. I remember when players were paying for SoKs, now established players have all but removed them and sold them all to the Antique Dealer. Who will pay for diamonds when the prizes will be obsolete when the next event comes around? Only the diamond miners will reap the rewards.
I think there is something to your point. It seems the players are starting to automatically expect/demand that each subsequent event offers a building ever more powerful. This starts to accelerate how fast past event buildings get obsolete.
Foe supposedly is a game that rewards a longer-term perspective, and having a well thought out strategy for advancing, even if there are numerous main paths --trading, framing, fighting, or a mix-- that a person can choose from.
If there is too much change and turn-over, the strategy part is somewhat undercut, making it less interesting, IMHO. Currently, part of the motivation to advance is to experience some differences in game play: army strategies may need to change, layouts need to be adjusted, etc. All of which makes it interesting.
Recently, event buildings come along every couple of months that seem to quite alter the game play in your same era: i.e. whether to fight or negotiate GE, re-balance production vs population buildings, redesign the city, etc. Much of this use to come from advancing eras.
So now, possibly, camping in an era to get every more powerful event buildings may be enough, and many folks may then decide to postpone advancing eras.
One idea from the US forum was to bring back a few historical quests, which would offer uniquely themed buildings. If these offered roughly the same benefits/power as other recent event buildings, that might reduce the current demand for constantly one-upping the power of each new event building. Historical events could offer interesting buildings with cool designs/animations, distinguishing them from those offered in the recurrent annual events. Earning some uniquely themed avatars would also be a draw. That could help keep interest high (and diamond spending, which Inno needs) and let event buildings increase in power much more slowly.