It's more like this.
When Trader A is long 1 lot then Market Maker is short 1 lot
When Trader B is short 1 lot then Market Maker is long 1 lot
Price goes 20 up
Trader A is up 20 and Market Maker is down 20
Trader B is down 20 and Market Maker is up 20.
But now lets expand on this using the ideas stated above.
20 traders.
Trader 1 trades Sells 15 lots, Market maker is Long 15
Traders 2-20 collectively buy 16 lots, Market maker is Short 16
Market maker needs to hedge his position and does so because he is now exposed 1 lot short.
Trader 1 has a direct line to the buy sell desk at OpenMarketOperations.Gov and knows when price is gonna move(so this trader doesn't need anybody to lose money for him to make some).
Traders 2-20 use a variety of different analysis and have to generally guess when price will move and where it will move too.
Government jumps in and shorts 200 lots. Price plumets until enough take the other side.
Spreads rise (Unless you have a fixed spread account). Market Maker is happily hedged. Trader 1 is happily profitable. Traders 2-20 are in varying states of emotion. Some blew there accounts. Some are on to the next trade already. Some are holding for a longer outlook. Some are on the revenge trade train(Newbies). Some have given up(Trading isn't for everyone).
In that made up scenario 5% made money, 95% lost, and the Market Maker could care less because it got the spread on 31 lots.
So your theory that one needs to lose money for the other to make doesn't exist here unless your broker really really sucks.