the idea that a twin is more eccomical than a quad or trijet isn't as clear as it may seem. Even though the tri and quad jets have more labour engine wise, experience has found that there comes a point where it is more eccomical to operate a quad than a twin. I know that when rodd eddington was Chief of Cathay, he said that cathay found that the crossover point was about 5 and a half hours. Keep in mind that Cathay's A330s, and its A340s are not configured for the same number of seats - the A330 has more. From what i can find, Airbus industrie clams in one of its product line reports that the crossover point between A330-300/A340-300 operations was just under 7 hours. There is obviously things to take into consideration about the intervidual operation - but i think it is safe to conclude this. The Quads jets potential is alive and well - i wouldn't totally rule out the trijet for the same reason.
Think about the current A330/A340 program. The idea behind it is exactly the same as the trijet widebodies of the early 70s. To try and offer the best combination between short haul and long haul operations. With Airbus Industrie, the idea is that you buy two different types of airplanes, provided you have a need for both types of operations. If you don't, you a suppose to pick the one that suits your operation best (ie either a twinjet or a quadjet). Thus, you didn't have a compremise between long haul and short haul. For the next larger widebodies - since the cost will be so high, maybe such a luxury is just to expenive, both in the design phase and for airlines to buy. So, the trijet may be the answer. If you have a trijet though, you can't produce versions of it with twinjets or quadjets and keep the same tail and wing. Who knows, but, i wouldn't rule out the possibity of a new trijet.