I am not convinced that the I-49 designation should be extended to the downtown ring. What would be the point? Even with stoplight removal the US 71 freeway will serve primarily local traffic, and I-49 traffic can get almost anywhere in the Kansas City metropolitan area from the Grandview Triangle.
Very interesting question. Two visceral roadgeek points:
(1) desirability of a "clean" 2di to 2di connection (I-49 to I-70) at downtown loop; and
(2) High Priority Corridor 1 is defined by legislation as Kansas City to Shreveport; a northern terminus at the edge of the downtown loop would seem to meet the legislative intent of "Kansas City" better than a northern terminus at the Grandview Triangle. Similarly, assuming parkway converted to interstate grade, uniform signage along the entire corridor as I-49 would seem to better define the corridor as being of national significance instead of changing the corridor's designation at the Grandview Triangle and signing an interstate-grade BWD as US 71 (since BWD has also presumably been legislatively deemed to be of national significance).
To be sure, roadgeek concerns would not rule the day. I believe the proper designation of the upgraded BWD would boil down to a political calculation: would the neighborhoods and businesses adjacent to an upgraded BWD be better served by an I-49 brand or a US 71 brand? My initial reaction is that, over the long term, local businesses would be better served by the I-49 brand, whereas the freeway's designation one way or the other would have less of an impact on residential neighborhoods.
Above said, it would be interesting to read a study on that question.