I think the birthday event is very balanced, and probably the best event so far because of the ability to upgrade previous reward buildings.
I'm a little surprised that people would ever delete rewards buildings, since they tend to be better than their equivalent "regular" building as much as two ages later. In addition, I wouldn't ever get rid of them, if only to show that I've been around long enough to have every event reward since the game started.
Why all the complaints? "Rewards will become obsolete!"? Perhaps people haven't noticed that one of the main benefits of the birthday event was to update previous event rewards. I don't think it takes a psychic to assume that future events will likely do this as well for other game rewards, including the King/Queen. I'm guessing this quest line chose the Halloween and Christmas rewards, since those are the oldest and most likely to be well out of date.
And all the other moans and groans? "Thanks for the worthless artwork!"? I think much of that artwork is spoilers of what is to come. It would be nice if perhaps the displayed Great Buildings were implemented as Stone Age buildings or some such (there SHOULD be stone age GB's - many impressive structures were built before what most historians consider the "bronze age", particularly in Mexico and Egypt). Personally, I find most gold/supply rewards almost worthless (except for the ranking points gain) since I have so much of both I can probably advance through the Progressive Era with what I have right now. Maybe some people would have been happier with a 1x1 building that produced 5000 happiness, a million each of gold and supplies every 15 minutes, and included a maid to come come over and clean your house once a week! Rewards have to be somewhat balanced, and I feel this event provided just that.
+1 from me for the Birthday event. Well done.