Let me pose this question another way. If GM is building the I6 in Flint, will they also start building the 2.8l 4 cyl for the Colorado/Canyon in Flint? It might improve the take rate of both diesels to have local manufacturing and not limit the total number of engines for the North American market.
The 4cyl is largely for use in other markets. Probably most popular outside the US. Plus, if it is [most likely] an entirely different family of engines, there may not be any feasible way to build both at the same facility. Remember, the Colorado is also being built in Thailand too. Given the fact that most of the diesels go into trucks built THERE, and very few come to trucks built HERE, it would not make any sense to make the engines here.
Given the one in Thailand is new or at least mostly new to take on the 2.8L 4 cyl I doubt they would scrap that and move production to a more costly site like Michigan.
The inline 6 is perhaps more targeted at JUST the North American market, which makes sense since that is where the Silverado is largely sold anyway. The Silverado is not built anywhere outside North America.
