My guess is that it all comes down to digging out some funding for I-57; since it's the "new kid" in the compendium of Arkansas Interstates, one wouldn't anticipate that the extension from Walnut Ridge to the state line would be somewhere in the queue -- well behind I-49 and the planned eastern extremities of I-69 (Monticello-McGehee). But the plain facts are that I-57 -- coming into the picture with 110+ miles of existing freeway has the advantage of being further along than either the central portion of I-49 or just about anything on I-69 except the 530 spur. IMO, putting up I-57 signage -- particularly the "future" type that has been mentioned, won't do much to evoke public sentiment toward finishing the corridor (I don't think this project could be "crowdfunded"!); it will all come down to available funding and how the state reps from that region can scrape enough of that together for this project.
IMHO, future-Interstate signage should only be installed when a corridor is going to become an Interstate, but does not yet meet Interstate standards. Otherwise sign it or don't sign it. I don't think it's going to hurt anything to sign the existing US-67 freeway as I-57 without some "future" thing, especially considering there are other full fledged Interstates with multiple significant gaps. In the first couple decades of the Interstate highway system there were big gaps all over the system.
I-57 may be the "new kid" in Arkansas Interstates, but it has some big advantages over both I-49 and I-69. It would cost a lot less time and money to finish I-57. There's a lot less new terrain highway to build and fewer upgrades along parts of US-67 and US-60 needed to fill the gap between Walnut Ridge and Sikeston. I-49 between Fort Smith & Texarkana covers a lot of difficult territory. And there's an expensive river crossing next to Fort Smith to fund before any of the mountainous segments of it get tackled. Southern Arkansas has less hilly terrain standing in the way of I-69, but The Great River Bridge is a hell of an expensive funding obstacle, especially with Mississippi offering no real time table on when it would be able to pay for its part. A completed I-57 might actually make the I-49 and I-69 projects less urgent (especially I-69 with its very crooked, indirect route).
The thing is, "I-57" isn't nearly as important strategically as I-49 is. There is an alternate all-freeway route between I-40 in North Little Rock to I-57 in Sikeston but there are no all-freeway routes between Texarkana and Alma unless you want to go all the way west to I-35 or all the way east to I-55, both which would be hundreds of miles out of the way.
The "alternative" all-freeway route between North Little Rock to Sikeston is a backward L-shape. I think a completed I-57 would be just as important a link as I-49, if not moreso. It would provide the most direct link between the Dallas-Fort Worth and Chicago metro areas, both of which have far more people than all the cities along the I-49 and I-29 corridors. Plus it would pick up a lot of NAFTA traffic. Right now traffic moving between Dallas-Fort Worth and points like Chicago are best served taking US-69 thru Oklahoma to Big Cabin then getting on I-44 to St Louis.