Attrition is a symptom of the problems in the game, not a cause. Merging servers might help pump up the numbers of players in each server, but it won't fix the root cause of why players are leaving.
For example, chat mode is ridiculously primitive. There are more than a dozen players on my PM list that I haven't talked to in months, if ever. But when current friends send me a message, I don't get notified. And why does my keyboard keep closing itself? Making it easier to chat with our friends would keep us around, chatting with friends, even when the game itself is incredibly dull.
I think this is a great game, but it could be much greater. Instead of focusing on new bells and whistles, like heroes and contests and ways to convince people to spend money, Gree needs to use some of that money to improve the basics of the game.
Also, rather than merge servers, I wish they would figure out how to allow players to jump up to a higher server level. Not down, because that would cause a flood of power players into newer servers. But if S5 isn't challenging enough anymore, why not let that account move to S1? Let the merge be voluntary, not forced.