I think that is the main reason for it, but there are also other systems there that do not have local variables (such as the DateTime extension), so I also think it can make sense to turn isolated systems into extensions, even when they do not have local variables. The advantage seems to be that there is less to compile and include in the resulting game binary. I also think it will be easier to turn it into an API selection with it being an extension.
That said, I don't think there is much difference to having it in the graphics system. I am mostly against it because it is more work for little or no gain as far as I can see, and the current system works

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In regards to the original question, I think it could make sense to turn the particles extension on by default. Should we do that?