I don't really see what you have presented as an "argument" against polymarket. [1] tells us that it has costs and benefits. Well everything has costs and benefits, but those are subjective anyway. That is not an argument against it and cannot be, because no science can tell you what is good or bad (i.e. what counts as benefit or cost), let alone whether the good outweighs the bad. [2] tells us that polymarket exhibits the same property as any other securities market, which is that only a very small amount of players tend to make a profit. Again, that is not an argument against it unless you want to include some kind of resentment or digust that some profit more than others.
Inequality of profits is both expected and unconcerning. The social function of the stock exchange is not to ensure everyone receives a fair or equal profit, but to ensure that the pricing of ventures and assets is correctly (as is possible) adjusted to expected future demand of consumers. For polymarket, the function is to express information about expectations of future events. There are straightforward economics reasons why polymarket and similar institution tends towards expressing our expectations about future events.