^ Relevant add-on to my rambling, lol ... I decided to look at the attendance for Illini/Northwestern games since 2000, as it looks like we are doomed to play them in November as our big rival in perpetuity. I looked at this by location to give some perspective, also. Attendance first with percentage of capacity and parentheses (and yes, I adjusted for our change in capacity by year and averaged it over the timespan).
CHAMPAIGN
Average: 41,569 (66%)
Maximum: 60,253 (96) in 2009 ... max for November game is 54,516 in 2007 (96% of capacity that year)
Minimum: 27,624 (46%) in 2021
NOTE: As noted above, this includes playing them twice during the warm weather months, where we drew 53k in 2011 (that awesome comeback!) and over 60k in 2009. Take those two out and only look at November, and the average goes down to 38,196 or 61% capacity...
EVANSTON
Average: 30,021 (64%)
Maximum: 40,658 (86%) in 2000
Minimum: 22,242 (47%) in 2006
CHICAGO: SOLDIER FIELD
Average: 33,514 (55%) in 2015
CHICAGO: WRIGLEY FIELD
Average: 41,058 (100%) in 2010
So, again ... my personal take is that this rivalry game on Thanksgiving Weekend is absolutely doomed and stupid if it remains a standard home and home. Northwestern will never carry attendance at their place (their genuinely great teams struggle to sell tickets), and Illinois is just in a unique situation to draw poor crowds in Champaign that weekend due to a variety of factors, one obvious one being over 70% of the student body heading back to Chicagoland several days before...
While we would all agree the 2015 showing at Soldier Field was pretty bad, it is still significantly better than the floor in Champaign or Evanston, both in terms of total attendance and as a percentage of capacity. It is difficult to compare Wrigley, as the 2010 was such a novelty with College Gameday attending and it being the first college game there in the modern era. However, it simply needs to be stated that people showed up.
CONCLUSION
I really am sympathetic to the idea of not giving up home games. When I was younger, I loved the idea of something like playing Mizzou in St. Louis, but I now appreciate the argument against it. However, I very much maintain that the Illini/Northwestern game on Thanksgiving Weekend played on campus is uniquely set up to fail in a way that many other rivalry games are not. There might be other schools that have a lot of students leave (e.g., Iowa), but they have a much more exciting rivalry setup (e.g., playing Nebraska on Black Friday). Other schools might have the same problem of struggling to find consistency in the last 30 years (e.g., Minnesota), but they are playing a bitter rival that their fans genuinely get excited for (e.g., Wisconsin).
SOMETHING has to be done to spice this up, and I personally think that if the Bears do build a dome on the Lakefront, this game has to go there every year via some deal that both Illinois and Northwestern agree to. While it will not reach the "event status" of something like Georgia/Florida in Jacksonville or Texas/Oklahoma at the Cotton Bowl, it COULD genuinely develop into a cool holiday tradition. With so much of our student body and alumni base in Chicagoland, especially over holidays like Thanksgiving, and a proven track record of Champaign and Evanston having abnormally awful attendance for this game, I think it is a no-brainer to set up a neutral site rivalry event in an indoor setting (i.e., nobody is turned away due to the cold) that can develop into something Illini fans get excited for every year.