I support this a lot!
Flight Rising is constantly looking for new Treasure (regular currency) sinks. Every suggestion that comes up on the forums tends to have someone saying “This could be a treasure sink if you make it something you pay for with treasure!”
Making sure the currency you can obtain freely in-game makes its way OUT of the economy and back into the game is incredibly valuable. In a few years, if there aren't enough Pebble sinks, then the value of Pebbles will plummet and Moonstones will be the only currency worth using.
Please consider reverting this change, and making expansions Pebble items. This does not mean the site gets less money. Players can trade Moonstones for Pebbles via the Flea Market, and so any player wishing to spend money to bypass the long wait time it may take to save up for a high Pebble cost expansion will still be able to purchase Moonstones to bypass this. I have purchased Gems several times on Flight Rising to exchange them for Treasure in order to buy Lair expansions.
Aside from the fact that this change loses a very valuable Pebble sink, it is also just unfriendly to users. The first thing a new player sees when they start playing any of these activities is that when they click expand they are greeted with a box that says ‘Pay our premium currency to expand this."
New players who don't know that this game allows for conversion of currencies may be immediately put off by seeing that the game requires a premium currency for expansions to activities.
I very much support more things to buy with Moonstones, but I don't believe expansions should be limited to Moonstones (at the very least, nothing but perhaps the last one)