It may depend somewhat on the degree to which Connecticut also pushes for the extension; A second state desiring an expansion of service is a rather different matter than North Carolina just trying to get off the hook. But I do think the plan has a reasonable chance, partially for the reasons you mention (cost to Amtrak is marginal); The Carolinian is arguably already a long-distance train in all but technical definition. As mentioned, beware however of what the so-called '750 mile rule' actually says rather than what you read on the forum.I heard at another board of North Carolina's plan to extend the Carolinian to Connecticut to not have to fund the train anymore (http://www.ncleg.net/documentsites/committees/JointAppropriationsTransportation/2017_Session/3.22.17_RailDivision_NCRR/3.Worley_NCDOTRail_Division.pdf, p. 9).
I'm thinking the chances of Amtrak falling for that are slim and non, especially with proposed Amtrak budget cuts. The Carolinian covers 95% of its operating costs by ticket revenue (https://www.fra.dot.gov/eLib/Details/L18616) so it wouldn't be that much extra a cost to the US and would be far better than any LD train other than the Auto Train. Of course then other states could try to follow suit so it would set a precedent I'm pretty sure Amtrak/Congress wouldn't want set.
And yes, there is the issue of precedence. However, there are relatively few state-supported trains which are even possible candidates (such as the Pennsylvanian, again to Chicago). Nobody is going to buy the idea that Illinois suddenly decides the Carbondale trains really need to run all the way to Mississippi.