It seems to me that if you're a non-controversial incumbent in Illinois, you stand a good chance at getting reelected, regardless of political party (Topinka always got reelected, albeit by narrow margins in 1994 and 1998). Munger stands a chance at winning the remainder of Topinka's term. There's no guarantee she'll be defeated. I'm not aware of her doing anything that would make Illinois voters angry. Granted, statewide races aren't held during presidential elections, so this will be new territory, but I wouldn't count her out at all.
Honestly more than anything else, it has to do with whether a GOP candidate is popular in the suburbs.
In the 80's and early 90's, the city was solid D, suburbs solid R, and downstate swingy. Then in the 90's and early 00's, the suburbs became more friendly for Dems but ILGOP was worried that if they nominated someone too popular in the suburbs, this would lose them downstate (they worried about this in Kirk's race).
That's proving to just not be true anymore. Downstate will vote for Republicans pretty much no matter what. Even Dick Durbin, popular downstate Democrat, lost big downstate in 2014 (although he did better than other Dems that cycle).
It seems that now the city is solid D and downstate is solid R, so the game is in the suburbs. Munger is a very good fit for the suburbs, similar to the likes of Topinka and Cross were.
She will have a shot, but I do also think that Mendoza will be a strong candidate. Not to mention that Munger is a unique incumbent - she's never appeared on Illinois ballots outside of the legislative district that she ran in.
