The NDC did not fall apart because McGovern was nominated, it fell apart because the political interests of WWCs and minorities started to be directly opposed to each other. That process began with the Watts riots in 1965 and escalated through to 1968. Many WWC RFK supporters went directly to Wallace in November instead of Nixon, which should have been a 5-alarm warning for the Democratic Party. They ignored it and didn't reassemble the NDC until 1992, when Clinton swapped out farmers for suburbanites.
Basically this. When the Democratic Party decided to completely ignore white ethnics, it lost its ability to govern as a party. If there had been a stronger attempt at a colorblind, rather than an intrinsically race based liberalism in the late sixties, this might have turned out differently. RFK could hold everyone together, and Humphrey also had a good shot at it.
Jackson of course would have been the penultimate Democratic candidate. He holds the New Dealers, labor, the blacks, the environmentalists, the hawks, and also grabs back the WWC. The New Leftists will probably sit out, but they were never a large enough constituency to get the Democrats majority status anyway, so a Jackson candidacy at some point (either '72, '76, or '80) would be a godsend for the Democrats.
The only problem the Democrats are going to have is coming up with a left-wing alternative to Monetarism as a cure for the bad seventies economy. Hubert Humphrey was proposing social democratic coordination and planning in the last years of his career, and the astounding power of labor in the seventies before it's flameout near the end of the decade might actually make something like that possible, if you get the right person in the White House.
Carter is not that person. He has no clear base in the Democratic Party and is by all accounts, a terrible manager of nearly everything he touches. You have to make sure he gets his ass kicked in the primary or that the primary deadlocks in '76 and the DNC nominates Humphrey as compromise to get the New Deal coalition to hang out for a bit longer. Or just have Jackson trounce Carter in the primary.