Epic will never have a monopoly as long as Steam exists. They aren't complete fools here, they realize this but are aware that even having a slice of that massive pie is enough to fuel numerous positions in the future.
They still wouldn't be a monopoly engine-wise, even excluding Unity you have Godot and O3DE as popular open source options. Ignoring that a lot of studios are now sharing internal custom engines, like Sony's studios with the Decima engine.
