Money gets created when people take out loans which creates debt which is then traded in the markets. That's not money flowing into the markets, that's just the markets doing their own internal bookkeeping (much like D&D uses HP for internal bookkeeping about character health).

If you want to convincingly argue that markets are positive sum you have to demonstrate that markets produce more than they consume in terms of something besides money. E.g. when some of the factory workers quit to go be day traders, the factory ends up producing more while consuming the same or less due to the positive effects of the investments enabled by the increased trading activity.

This might be the case in some cases, and it won't be the case in others, and I don't know how to sum it in aggregate. Presumably somebody does.

Whether it comes out positive or negative, asset prices are an implementation detail, not the thing we're measuring.