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EDIT: I changed title from free to play, to having unlimited free trial because people can't be arsed to read.
NOT free to play, unlimited free trial.
The quickest way I can say it without you shooting me is that there are members and there are non-members, I'm not talking about a cash shop. Some pay, some don't. Oh god put the gun down Mark!
I know what you're thinking, pay to win. I know that feeling of thinking, maybe that guy has an advantage because he spends more money than me. Thats why I say subscriber based free to play. Same model that Mark already described, just with an extra range of free. No buying diamonds. No buying station cash. No buying boosters. You're either a subscriber, or your not.
Assuming I've calmed your fury, let me explain why the game should be free. As games age they lose customers, especially niche games. I currently play planetside and most people play it for free or are at least not members. I'm both a member and only play with a 50% boost up. I'm one of the whales that f2p's rely on, and let me point out that most of us here, assuming we will fund the kickstarter, are whales as well, we're just whales by a different premise.
In games where the entertainment is provided by other players, and if there isn't entertainment then people quit the game, then its worth allowing people to play the game for free, if for no other reason than to provide entertainment to your paying subscribers.
I know what would happen to PS2 if they made it no longer f2p, they'd lose most of the playerbase and that would provide the remaining customers to lose interest. A similar effect would happen if they changed it from f2p to pay2win, and they are very different and specific to the game at hand.
Lets take a game we're familiar with and apply f2p subcription funded. Daoc subscribers continue to get the same thing they do now, nothing changes. Though a free player would see inconveniences introduced.
Hell you could even put your free players to work for you and have them solve problems that you can't solve yourself. As a developer you can't tell you paying customers not to play on a realm and server thats too overpopulated, but you can tell your free players to go play a realm and server that needs them.
And of course don't progress as fast.
And get extremely limited access to the game until they get a sponcor. (just someone to vouch for them not being a gold farmer or cheater)