Not sure if I remember this correctly. Wasn't the game was at risk of fading into obscurity early on and likely would have been canned long ago but the massive popularity of the Elin and all their cute/sexy purchasable outfits kept the game going.
Or was that just in Japan? Again, I don't really remember.
Yeah, while I don't know the numbers I can imagine that going from a subscription model to F2P back in 2013 is what kept it going to this day, and Elins likely stood for a large chunk (if not the majority) of the microtransactions by virtue of being cute anime grills.
That said, the "sexy" portion of the game ended up being one of the reasons why I dropped it back in the day, and I'd say that aspect is a double-edged sword - appealing to boners naturally results in a lot of impulse spending, but having every other female character running around in their underwear is going to push a good chunk of players away, even more so when one of the races are, well, lolis. So many good outfits ruined by constant pantyshots...