US-57 from Eagle Pass to the Y-junction with I-35 in Moore? Nah, I'm not feeling that for an Interstate corridor. Standard 4-lane divided with at-grade intersections maybe. Even that might be a stretch. The only thing I think helping this concept at all is the relatively short distance of the route (around 96 miles). I think Laredo to Corpus Christi is a far easier potential Interstate corridor to justify building. I-69W would at least cover half the distance to Freer.
Eagle Pass is a significant border crossing, but it pales in comparison drastically to the Laredo crossing. Laredo is on a direct path between Monterrey (5 million metro pop.) and San Antonio. Mexico Autopista route 85D is mostly limited access, pointing into the I-35 corridor. Route 57 going up to Piedras Negras/Eagle pass is a mix of 4-lane divided and 2-lane, but not limited access. If truckers are looking to bypass all the traffic going into Laredo they'll be more likely to take Route 29 up to Ciudad Acuña to cross the river at Del Rio.
An I-27 extension that goes down to San Angelo, then down to Del Rio, Eagle Pass and Laredo would be more valuable than this US-57 idea. It would be even more valuable still if San Angelo and Abilene were linked with a 4-lane divided route. That would provide a higher quality connection into the I-44 corridor. Trucks could more easily bypass Laredo and all the I-35 traffic in Texas by going thru Del Rio. IMHO, I-44 should be extended down to San Angelo to meet an I-27 extension there.
But, yeah, TX DOT has
lots of different fish to fry.
The I-69 corridor is one priority. Progress is slow-going. But if they can get finish enough segments in/around small cities like Lufkin, Nacogdoches and Marshall it may create enough momentum to finish more rural segments faster.
The I-14 corridor has its own uphill battles. Even though there is no Interstate-halo floating over the US-290 and TX-71 corridors those two routes are going to end up getting more and more limited access spot upgrades by virtue of all the growth in Austin and Houston. I can easily imagine both corridors being 100% limited access before I-14 is substantially complete within the Texas Triangle. The TX-80 and TX-46 corridors are important connections to I-10 for the rapidly growing cities of San Marcos and New Braunfels. Buc-ee's is planning its biggest ever store in Luling, where TX-80 meets I-10. I've gotta keep beating the dead horse on US-287 from Amarillo to Fort Worth.
With all that going on, the concept of US-57 as an Interstate from Eagle Pass to I-35 just makes me go: huh?

I-12 in Louisiana would like a word with TxDOT.
I-74, I-76, I-84, I-86, I-87, I-88. I-49 remains disconnected even though there are long term plans to connect both segments eventually. I-57 may exist in a similar manner for some time. I-69 has multiple disconnected segments. So it's not going to hurt anything for Texas to have a much longer and more significant I-12 route that doesn't connect at all to the one in Louisiana.