People are already paying real money for this game. They could go to any other private server to get free Members content. So obviously there's a draw to stay here. Probably for multiple reasons, including keeping this server close to original.
Also, the obvious push is to have a majority of the player base using the P2P world. The separate world is essentially a way to gate keep the content so people are forced to pay for it. They're already paying to play this game for a F2P experience. Which in itself is absolutely wild. Now - the point in raising the actual real life cost over time is to force people to sink massive amounts of GP into it. Which would balance the economy.
I mean, imagine if people with 5 stakers were forced to pay 5m GP a month in order to use them all. Or guys who want 5ip's in the wildy, they can have that, but for the cost of 5m a month gp. Its a way to keep these activities more balanced, keep the economy balanced, and since everyone is already paying money to play the game, why not just put the good content behind the paywall so that they will pay for it? I mean, most users are already buying subs so they can increase exp rates. They obviously see value in this server otherwise they wouldn't spend money here. Point some of the money spending toward the bigger experience. If they're here already they aren't going to leave or they already would have. Who sticks around and spends money on F2P content you can get on 50 different servers? Allow people to purchase their way in or spend their way in. Its a pretty easy concept.
Ultimately, This game is built to burn itself out by design. There's no way to stop it. Eventually every server hits maturity/hyper inflation, especially one that is aiming to stay true to the original experience. With increased exp rates and QOL improvements that process is only up. Why not try something different instead of doing the same song and dance that has been done by 20 other private servers over the past 7 years?