I agree with the general theme of your post. I've lived in Chicago for 11 years and went to our game @NIU so I feel I can comment with some accuracy on them. The Big Ten agrees with your post too because they added tiny population Nebraska when they could have chose massive markets instead.
Illinois and Northwestern don't even have the Chicago market. Iowa is a much bigger deal here, TWO new "Iowa" bars just started promoting themselves in my neighborhood alone, they have fricking billboards promoting themselves. ISU fans won't want to hear that, but the Hawkeyes own Chicago more than local teams do.
My take on the market:
1. Bears
2-100. Bears
101. Michigan
102. Ohio State
103. ND
104. Iowa (when Iowa is good they jump ND lately)
105. Wisconsin
106. Rest of B10 including the two Illinois schools
107. NIU
That said, I'll defend the people who bring up NIU to an extent. They are far more attractive than any of the other MAC schools. Their games are always on either 670 or WGN 720 radio, both stations completely dwarf any radio coverage that a Non-Texas team gets in the Big 12, especially 720. Usually about half of their games are televised on regular local TV in Chicago with no cable or dish needed.
They're a massive step down from an addition like BYU, but if people are considering Memphis they should consider NIU. For example NIU would easily be better than Colorado State that has been mentioned.