I think it would be better to have train link connecting Kathmandu with the proposed international airport, rather than this road. What about Pharping-Hetauda and Banepa-Bardibas  roads? Don't know the details, but Pharping Hetauda road is perhaps operational now, they could have upgraded it. There also used to be rumours about Bagmati corridor, and KP Bhattarai used to talk about some tunnel link. Maybe there are lots of options. Also, seems that the economic activity along the Prithvi Highway will have the fate of the Tribhuvan Highway in a few years, or maybe for travel upto Bharatpur, it might remain preferable.