Nah, it won't have a monopoly. UE4 is a big deal for a lot of reasons, particularly its graphical quality. It's not the friendliest for indie devs at the moment, but it's been evolving and improving very rapidly.

And we'll see how long it takes Unity to actually support the current C#. My guess would be a couple years, since Unity 5 isn't even out yet, and there's usually 18 months between major releases.