whoops accidentally cut off my comment anyway you could still earn a by allowing custom pads to play on .
Anyway there's a number of different ways to make money without destroying your player base(customers) like akanton said
in addition to what akanton said
look at Riot games marketing scheme of league of legends
They made a great balanced game without you having to pay a cent but to get skins you have to pay.
now obviously the "skins" part wont be the major income earner in this game(but you could still earn a bit by
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