There are many things that factor into how likely HSR is to be used. Intercity travel demand is one of the chief metrics, but you also have to consider the quality of intracity public transport. Indianapolis and Columbus as a city pair simply does not score very well on this metric; you'd put the billions of dollars investment to much better use elsewhere.
The point about collinearity is that you can make a strong demand by combining many individually weak demands. And this is the problem in the Midwest: the strong demand is Chicago, and it has a lot of moderate demand endpoints, such as St. Louis, Minneapolis/St. Paul, Indianapolis, Cleveland, Detroit. But the real strong city pair is Chicago/NYC, which is already at the extreme margins of acceptable time, which means substantial detours to hook up the lesser demand centers is going to kill ridership.
The point about collinearity is that you can make a strong demand by combining many individually weak demands. And this is the problem in the Midwest: the strong demand is Chicago, and it has a lot of moderate demand endpoints, such as St. Louis, Minneapolis/St. Paul, Indianapolis, Cleveland, Detroit. But the real strong city pair is Chicago/NYC, which is already at the extreme margins of acceptable time, which means substantial detours to hook up the lesser demand centers is going to kill ridership.