I don't think that any of the 2018 Democrats were DOA. All three ran really, really bad campaigns against strong, well-funded opponents - and still won roughly 45% of the vote each. Yes, 2018 was a good year for Democrats at the national level, but the "wave" didn't reach everywhere.
Democrats could win any one of these seats again with a decent candidate, a strong campaign, and a favorable national environment.
The Obama-era southern Dems on the other hand... I'm not sure that anything short of setting themselves up for party switches years in advance would have prevented those losses. Only a Roy Moore level candidate could make these seats competitive again in the immediate future.
I absolutely agree with you.
Of the three (ND, IN and MO) I would think Missouri would be the most likely to elect a Democratic Senator. McCaskill also had the issue of being very unpopular.
Geography hurts Democrats in Missouri but it can also
help. It could be pretty efficient to run a metro based campaign and be able to earn a statewide majority. (STL, KC, Columbia and Springfield).
I do not believe we cold win North Dakota any time for the foreseeable future. I feel like Heitkamp got unlucky with Kavanaugh and also screwed up her own campaign.