So argues Vox, whose function is to argue this to the exclusion of everything else. This piece is structured in such a way as to present a half-criticism of Clinton: sure, she is certainly "well-positioned to win both the primary and the general election," but she is too centrist for her position to be the long-term one for the Democratic party.
The analysis finds that Clinton relies on blacks, labor unions, and older voters for her support. But blacks favor politics well to the left of others, as do labor unions; and as do the young, whom future elections will have to rely upon because the older voters will die off.
The obvious problem with this analysis is that the immigration Vox often champions is diluting black as well as white percentages in the electorate. We hear all the time about how white voters are becoming less important, and white preferences less decisive. But that will be true for black voters as well, especially in the context of Democratic politics and especially if Vox is correct that Latino voters will continue to trend Democratic.
Union labor, meanwhile, is like all American labor being strangled by competition at home from mass immigration, and competition from abroad by globalization. They won't be as large a bloc either, and they may be up for grabs if the Republican party takes a long-term populist swing.
As for the young, well, the young are nearly always liberal while they are young. Remember the 'older, more conservative' Democratic cohort Clinton is leaning on are the Baby Boomers. If former hippies drift into economic centrism as they age, what confidence should anyone have that today's young will not?