OK, let’s break this down.
First of all, in order for all individual account holders (what you’re calling retail traders) to be winners all of them would have to be trading in the same direction. If even one person trades the other direction then someone is losing.
So basically, it’s impossible for every individual account holder to be a winner all at the same time.
To the extent that you have individual account holders (IAH) taking matched opposing positions you have them in a tug-o-war between themselves as each side’s gain is the other’s loss. Things are rarely in perfect balance like that, though, which means there’s some fraction of IAH positions not offset by other IAH positions. The remainder are offset by the liquidity providers (broadly speaking). This means we have IAH trading against their fellow IAH on the one hand, and IAH trading against liquidity providers on the other.
Drilling down on the latter, the liquidity providers will accept a certain degree of exposure to IAH positions based on their risk management policies. Anything above that will see them look to hedge away, which basically means unwanted exposure aggregates up to the big inter-bank dealers at the top of the food chain. They, of course, will have their own risk management procedures to tell them how much of that exposure they will accept and what they need to offset. That bit at the end is what you’re talking about with what you say about the losses being passed on to the hedgers.
The willingness of brokers, market makers, and liquidity providers to accept some degree of exposure to IAH positions does two things. First, with respect to this discussion, it means there can never be a full offset outside the retail forex market for the net IAH exposure. In other words, the liquidity providers, et al will never have a matching gain from the hedger in the inter-bank market because they won’t actually look to offset that full amount. Second, this serves to severely limit the impact of retail forex trading on inter-bank exchange rate movements.
But we do have [I]some[/I] liquidity provider exposure offsetting in the inter-bank market. With respect to that, your observation about the hedgers is a valid one. I do not make the claim that forex overall is zero sum because any market which involves the actual exchange of assets - as opposed to contracts for a future exchange - is not functionally zero sum.
Retail forex, though, is not an asset market. It’s a obligation market with marking-to-market, making it a zero sum portion of the broader forex market. The same is true of forex futures. Just because my gains come at the expense of trading profits Citi makes outside retail forex doesn’t change that any more than my gains coming at the expense of your poker winnings would.