Hawley is not a pushover for McCaskill - he won by more than 20 points last year, running ahead of Donald Trump. McCaskill is doomed or very close to doomed against either Hawley or Wagner.
past election victory margins afford virtually no guarantee of future election margins. Koster won his attorney general re-election in 2012 by double digits but that did not save him in 2016.
The candidates and the campaigns Democrats ran in MO last year was literally the most pathetic I have ever ever seen. Even Koster's campaign was quite pathetic past labor day. He got way to over confident and not on the aggressive enough. It may not be entirely his fault he lost but it is his own fault he lost by six points.
Kander was really the only Democrat who ran a good campaign but was brought down by the rest of the ticket and winning a Senate race would already be an uphill climb.
so basically, no.. McCaskill would not be doomed against Hawley and I think Wagner would be stronger against her.