The lift fan concept is certainly viable, but has not taken off (pardon the pun) mostly because the higher disk loading means that the engine power must be at least twice that of an equivilent helicopter, if not 3 times. This means more fuel burned, greater initial and operating cost, more expensive overhauls and much lower payloads. The tilt rotor barely works, and it requires only 50% more power than a helicopter, what does that say about a lift fan that needs 2 to 3 times the power?
Additionally, there is no autorotation capability, tolerance to engine failures is very poor and the control has (in the past) been much more marginal than a helicopter.
Let the market decide!