Because it's pointless to extrapolate on what if scenario. What type of thing would you have in its place, closed, semi-closed, open primary? Early ballots? Then we have to account in the fact that the Clinton camp would direct more resources into those states if it were the case. Instead of a blowout 70-30, you can have a Missouri or a Wisconsin. Either case would not significantly benefit Sanders.
I disagree with your method. Obama carried the Southern states and he did fine in the general, twice. African Americans are the most reliable voting bloc of the Dems. Why would their voice be worth less just because they live in (historically recently) red states? In this election, there are potential for the Dems to win MS, SC, NC, AZ and GA. All red states, 4 of them Sourthern.
Party's platform, constituency and geographical advantage change over time. Just as the GOP boxed themselves in with the Southern strategy, you don't want to repeat the mistake with Dems and the North and Midwest.