When I was a co-op student in the division of Planning of what is now INDOT back in '76, I remember hearing about an ambitious plan at the time that would had decommissioned or moved to the interstates many US and state highways that paralleled Interstate highways. US 40 would had lost its independent alignment throughout Indiana, for example.
Of course, that plan never came to frution, and I am not sure how serious it really was. I supsect it met or would had met a buzzsaw of opposition from local officals, not wanting to suddenly have to maintain all of those roads.
To me, old US 40 inside Terre Haute, as well as Washington Street in Indianapolis should be designated "Historic US 40" with signage provided, but with local control and maintenance. I don't expect this to happen, but it would respect the historical importance of the original alignment of US 40, having been the National Road.