If you spin the prop slower, the plane is going to go slower, which is unacceptable.
As Jesse says, Keep hope alive.
Me, being a realist, know that the UDF is dead. To prove my point, here is the the GE UDF test bed aircraft crashed for a movie prop. No Air and Space Museum for it. After the first flight, everyone pretty much knew it had no future.
I personally think it sounds cool (and if there were only a couple of them, I wouldn't mind), but it aint 1944 anymore.
One of the reasons why the ATR-42/72 turboprop airliners switched from four-blade propeller units to six-blade propeller units was the very fact this allows for slower fan blade speeds for the same thrust output, which 1) reduces noise quite a bit and 2) reduces fuel burn due to slower speed of the engine during flight. Why do you think Lockheed did the same thing with the C-130J, the current model of the famous Hercules cargo transport?
Hence the reason why the switch from eight-blade to ten-blade propfans. With more blades, this allows for slower blade spin speeds with all the benefits I mentioned above.