Let's take two kinds of players. One that does not want to spend real money (let's call him "the wolf"), and one that does spend money (let's call him "the fox"). There is the main currency of the game world, one that has unlimited drops. In our case, those are crowns, and a secondary currency that only comes to existence when the fox buys them. In our case, energy.
The fox can spend money to get a large pool of energy or could spend money to get items that only foxes can buy. However the fox can never buy crowns. That's where the wolf comes in.
The wolf is willing to fight on hours end to get a ton of crowns. He however does not buy energy with real money, instead he'll exchange it for crowns. He does not want to buy prize boxes, but wants the items you get from them, so he'll buy them from the fox with crowns.
The foxes in turn relies on the wolves in order to get their crowns and the wolves will stick around knowing that they don't need to spend real money to get energy and fox items. It's a tough balance, lose one of the two and you will also lose the other.
If you make the items dropped from the prize boxes more useful than the ones you can get regularly, then wolves will either turn into foxes or leave entirely if they don't want to become a fox. And if you lose the wolves you lose the foxes in the long run. Goodbye player base. That's why the foxes items are never necessities but are still shiny and worth it enough that wolves will still want them.
The secondary currency, i.e energy, is another story however. Energy can be a necessity if the wolf needs it be. This allows for the fox to have a more steady income of crowns, than whether or not the item they have is shiny enough for the wolf to want. The price for the wolf to get energy is low enough and is energy useful enough that the wolf will most likely always exchange his crowns for energy. And that expansion that can be bought with energy is very tempting for the wolf.
This is also why the prize boxes have a chance to drop less favorable items. A fox may not want that item, but a wolf will think it's worth any amount of crowns you sell them for. In that case the fox still get's his money back in the form of crowns, and the wolf get's an item that he otherwise couldn't get without the help of a fox.
Both the fox and wolf are happy, and the devs will always get their money fairly, since even the non-paying wolves will still be helpful and make the game more inviting to foxes who will buy, and the economy stays alive for a very long time.
Of course, you as a fellow player are not a wolf or fox. You are a mixture between the two. You may buy only a little or buy a lot or not buy at all. That coupled with events and ever changing deals makes no one fully a wolf or fully a fox. Either way, know that MMO economies are complex and know that you are apart of one such economy.
If you ever wonder how a game as fair as Spiral knights is free, and continues to stay free. This is why. A balance elegantly being held! Any thoughts? Leave them below!
Your post has some good insights, but I would make a couple of changes.
I had trouble keeping track of which one was "wolf" and which one was "fox". I recommend instead "F2Per" and "P2Per".
The P2Pers don't entirely rely on the F2Pers for crowns. P2Pers also get crowns from playing. Toward the end you concede this point, that most players are a mix. But it's too late for me, because...
The real value that the F2Pers give to the P2Pers is community. Without a large crop of F2Pers, the P2Pers would have fewer friends to party with and a smaller audience for their fancy costumes. (Of course, a really big game is strong enough to skip the F2Pers entirely, but that is not Spiral Knights.)