I think an I-2 extension to Laredo is indeed a very possible and justifiable thing. Just look at the population numbers in far South Texas. The Rio Grande Valley MSA, which includes Brownsville, McAllen, Harlingen, Mission, Edinburgh, Pharr, Weslaco, Rio Grande City and Hidalgo has over 1.3 million people. South Padre Island is growing in popularity as a tourist destination.
236,000 people live within Laredo's city limits, which is very close to the 249,000 population level of Lubbock -and that city was apparently big enough to warrant I-27 being built from Amarillo. Laredo is an important border crossing for US/Mexican commerce.
There are other border communities growing along the US-83 corridor South of Laredo, like Zapata, Roma, Escobares, etc. Regardless of what happens with I-2, the US-83 highway will have to be improved more and more as population grows in that region.