It would be a waste of money, which would be better off spent on widening I-95.
The fact that NCDOT has yet to publicly route I-74 south of Bolton is a strong indication they agree with you.
The solution is clear.... I-74 can follow US 74/76 all the way to I-140. At that point, it can change directions (a la I-69 in Michigan) and suddenly become I-74 SOUTH and take over I-140 to US 17. It can continue along the US 17 parallel route and then pick up SC 31 as planned.
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This requires no change in legislation... it still traverses North Carolina, and crosses the border into South Carolina where planned. It just requires the disbelief of I-74 making a right angle turn of sorts, and becoming a NORTH/SOUTH route to its end south of Myrtle Beach. The precedent for changing direction exists already with Michigan's I-69, and the precedents for ending near an ocean are many... and even in the middle of the country (I-27, I-44, I-39) not at another interstate.
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No need at all to make it an I-x74.