I don’t think he’ll win the primary, but obviously if he does, he’ll get to be a Senator despite... well, everything.
He'll be the frontrunner going into the primary, dems will see him as their best shot of taking the seat, he'll lose, whoever wins the primary will poll competitive with the democrat anyway, win by double digits, people will conclude that Greitens would have won the general if he had won the primary so it's a damn good thing he lost.
Kansas senate 2020 all over again.
If "whoever wins the primary" does not happen to be Eric Greitens or Roy Blunt but is still polling competitive with the Democrat, that would likely suggest a very underwhelming cycle for Republicans even if they outperform their polling by close to double digits in MO. Then again, you’re right that people shouldn’t place much faith in polling in general, even when they factor in likely biases.
