I've tinkered with Unreal Engine 4 a bit, and I've been somewhat disappointed in the performance. On my GTX285, I was getting ~30fps or less on even the mobile app examples (super simple geometry, like blocks on a table. Things that should be getting like 1000fps). Maybe it can be better optimized for this type of game, with the option to scale up the graphics quality for higer end machines, but I doubt it will ever be as fast as the Quake2 engine on mid/older aged hardware.
Also, the way the renderer works is great for slow moving/static scenes where it has time to calculate all the reflections/environment/global illumination type stuff, but with rapidly moving scenes, things tend to be very blurry (especially at low framerates). This makes identifying targets and such while rapidly moving through a map more difficult, which I think will make a lot of people unhappy. Also, small, fast-moving projectiles are extremely difficult to see. It might be possible to avoid this by avoiding the physical styled lighting, but I'm not sure if that's a trivial change, and it would come at the cost of a visual quality/realism (which is the point of updating to begin with).
The downloads would be very large. To get the kind of quality you're probably imagining, it would probably be a couple gigs for just the base install, and the maps would be much larger as well.
And then there's the time it would take to develop...
