You probably know this already, but I'll provide some more context for the interested:
In reality, two parties can walk away from a trade believing (in the moment) that they got the better deal. Otherwise, they wouldn't be trading in the first place. This results because people have different utility functions. A farmer might be willing to buy insurance that gives him negative expected value, because his utility function incorporates a larger risk term than the insurer. A fur trapper sells furs to a buyer because 1 pelt is not scarce to him. With derivative contracts this analogy gets a bit abstract, but the justifications hold.
Liquidity providers add liquidity to the market precisely because they think that the rebates are "worth more" than the liquidity they are providing. A liquidity taker might still fill their trade because they have a different utility function. The net gain for society would be the net gain in total utility, if we were somehow able to convert them into normalized units.
However, if the liquidity taker is a nearly identical firm with a nearly identical utility function, then that implies that the liquidity provider and taker disagree over who is on the losing side of the trade. One party has better information or luck than the other and the future eventually reveals which was the better choice.
To put it another way, I believe it is a zero sum game if players with identical utility functions (i.e. two small prop shops with $200K book) trade with each other. Both probably have identical utility functions, and future events will reveal whether buying or selling was the correct choice in terms of utility. There are quite a number of such players swimming in the market, so there is a zero-sum game of "who knows more" that goes on underneath the actual net utility provided by liquid markets.
In reality, two parties can walk way not just believing, but actually getting the better deal. Both parties can be winners, there is not always a loser when a trade is made. One guy could be exiting a long while profiting taking while the counter party is opening a short, and they can both bank profit from the trade. Maybe one is hedging and doesn't mind if the trade goes against him because it's hedging another trade in his portfolio to keep him market neutral. The idea that one guy wins and one guy loses is simply far to simplistic.
I mean, trading in paper without any insight into the capital allocation the paper is abstracting, and observed over a very short time horizon, it's more or less zero-sum. Many synthetic products just distribute risk differently. Etc.
But finance more generally is helpful for efficiently allocating capital towards wealth generating industry - and by industry I mean it literally, people physically doing things, creating things, generating wealth by the sweat of their brow. When you own a share of a wealth-generating enterprise that grows in value, you're acquiring some fraction of the discounted future returns on that effort today. This isn't zero-sum; it's a slice of tomorrow's wealth, today. And it's backed by real action, actual things, products and services that you can use or exchange with other people for things you prefer more. These are things that didn't exist before the capital allocation, and may never have come about without it.