Repulicans cannot win statewide elections in Illinois anymore. The dems will just run more people as they won’t have to worry about small districts being red.
That strategy would actually work AGAINST Democrats. If there are 17 House seats and the Democrats run exactly 17 candidates, how do you ensure that the ones at the bottom of the list (say, #11 through #17) will garner more votes than the top Republican candidates? And if the party runs more than 17 candidates, that simply dilutes the support among more candidates and makes it less likely that some of them will get enough votes to be in the top 17.
What really happens in an at-large voting process with multiple seats at stake is that political party affiliation becomes less important. That's because a minor party candidate like one from the Libertarian Party or Green Party doesn't have a chance in hell of winning a statewide election for governor or U.S. Senator, but could easily get enough votes to be ranked among the top 17 contenders among dozens of candidates.
That last point is important. I came across some voter registration information for Illinois that breaks the party affiliation down as follows: Democratic (48%), Republican (33%), and unaffiliated (19%). If candidates ran in a free-for-all election in Illinois with the top 17 candidates are all winners, then this would roughly translate to Democrats (8 seats), Republicans (6 seats) and Others (3 seats). Right now, the current Illinois delegation is split 13-5 in favor of the Democrats (the state currently has 18 seats but will lose one starting with the 2022 election). What the hell do the Republican Party and its supporters have to lose if the system was changed the way I described?