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None of the arguments I've seen SS make against releasing his source make much sense. Others stealing credit? People mocking his spaghetti code? Nintendo goons? They're all easily countered, and it just doesn't work for me.
I think one of the biggest reasons he doesn't want it to go open source is this right here.
This menu is present upon pressing Select while choosing a starter. With this menu, you can input a password to generate a "special" starter. These passwords are given away as tournament prizes and I believe as Patreon rewards (I know a level 100 shiny Swinub is a donation intensive, for some reason, and I believe this menu would be how to claim it).
Once the source is released, these passwords become ultimately worthless, since anyone could just figure out how to generate whatever they wanted. And just like that, there's no real incentive to participate in events or donate at all.