I would not be surprised at all, if the reason behind these balancing changes was - once again - spire. Until spire was released, events provided all rewards together - buildings, instants and diamonds (as WWs). Well, spire was introduced, and as they stated many times in the past, they are trying to develop the game so that every feature has a unique rewards to offer. Because spire took instants and diamonds, to make it a unique source, these went out of the events, where building-only prizes remained. Hovewer, this appeared to be neglibile prizes for many players and as they didn´t want to let spire take profits from the events, step for a change was needed, dividing in 2 possible scenarios. Increasing stats for new buildings (what would, after several events, get whole thing out of balance again) or decreasing stats of older ones. The latter won. That´s how I see it.
And honestly, I don´t feel angry at all, even though I own one of those touched set buildings. What looks bad today may look better tomorrow, I would wait which steps will be done next.