The national party made it clear Illinois was a write off well before the primary, which is why there were so many self-funded candidates in the race. They reiterated that fact when Ryan embarrassed us all, and the state party had to find a replacement. If they had been prepared to put money in the state we would now be several weeks into the campaign with a home-grown replacement.
You're undoubtedly right about where the blame lies. I've seen it myself in an important race, where the national GOP wrote off a candidate because he was against a popular Democrat incumbent, only to see the GOP candidate lose by a hair. They regret it now, I'm sure!
That race was important, but not as important as this Senate seat in Illinois. If what you say is true, it's really boneheaded they didn't take it seriously before now. Then again, often they don’t step in until the general election.