Sorry, very badly phrased by me. I'm trying to say 'zero sum - so what?' and 'not zero sum - so what?' I regret writing the second part, I was trying to say (badly) that any simple model of a market could be zero sum. You can add in more things to the model (commission, whatever) and say now it's not zero sum. And you can add in opportunity costs and claim it's zero sum again. But who cares? It means nothing.
Now I've unintentionally started an argument about what is / is not zero sum and what bits of a market you take your definitions from. That was the exact opposite of what I was trying to say. If it is zero-sum, why does that even change anything or make the trading (from the original paper) worthwhile or not? I'm trying to say that 'zero sum' or not, it changes nothing and gives no additional insight.
Zero sum markets are not really a thing. The normal phrase is 'zero sum game'. Buying and holding stocks is not a zero sum game - you make money. Trading stocks between speculators where the trading commissions exceed the dividends and long term market appreciation is a zero sum game. Same market, different games.
Now I've unintentionally started an argument about what is / is not zero sum and what bits of a market you take your definitions from. That was the exact opposite of what I was trying to say. If it is zero-sum, why does that even change anything or make the trading (from the original paper) worthwhile or not? I'm trying to say that 'zero sum' or not, it changes nothing and gives no additional insight.