"Yeah, but the CA results point to a 2018 environment (see: CA-GOV 2018), and Reynolds won by 3 points in 2018, so Finkenauer wins after four years of growth in the Des Moines metro and the Cedar Rapids-Iowa City Corridor."
In all seriousness, this is beyond Safe R with or without Grassley. Even if Grassley loses most of his crossover appeal to Democrats and D-leaning independents (which is actually somewhat likely), it clearly won’t matter in terms of the binary outcome. Not that this should come as a surprise to anyone (I know it will, but I said "should").
The # of Republicans in IA is getting to be too much for Ds, you're right.
Just look at the new IA-GOV numbers. Republicans approve, Dems disapprove, and Indies are split down the middle. But Reynolds is still +10 above water because there's just more Reps.