US59 will need to be upgraded to I-369 from Tenaha to Texarkana just for the truck traffic. Most trucking companies pay drivers by the mile which means they pay for the shortest route not the fastest. The shortest route from Houston to Texarkana is US59. I see this in East Texas with several trucking companies sending trucks from Texarkana to Waco/Austin/San Antonio. They send them via US59 to Linden, TX155 to Tyler, TX31 to Waco, then I-35. It isn't the fastest route, but it is shorter by about 30 miles. I-69 in MS, AR, LA will never have much traffic if it is ever built, too far out of the way. Any money, if it ever becomes available, should go to widen I-30 and I-40 in Arkansas.
If going up I-49 was the best option, they would be doing it now. US79 is straight shot as a two lane from Carthage, TX to Greenwood, LA, making it an interstate would not shorten it. Upgrading US59 to interstate standards will not greatly increase the truck traffic since most trucks are already using this route. The only thing that will increase traffic on this route is more trade in and out of Houston, not being an interstate.