“Steam, for all its shortcomings and 30% off the top, is the de facto platform for PC gamers, and developers have shared that when their game is available on both Steam and Epic, the latter makes up only a small fraction of sales.”
Epic will waive its 12% cut for six months if a game stays off Steam. That trades 70% of a big pie for 100% of a much smaller one. The old exclusivity deals at least paid cash up front. This one pays 100% of whatever trickles in and asks the developer to take the risk.