FWIW, I'm a grad school alum and didn't attend undergrad there.
Right now, in my opinion, the sidewalk "ban" is a means of defending a small fan base. In all fairness, graduating approximately 2,000 kids a year won't grow a fanbase very quickly organically. Especially given the large international component that doesn't even follow the sport.
As NU gains more and more national attention (as it has been slowly doing under Fitz and soon may under Collins in hoops), you'll see that mentality change. It will be a very small minority who stick to their guns about sidewalk alums, IMO. The school itself already attempts to attract non-alums through its "Chicago's Big Ten Team" campaign.
That said, NU will never have a fanbase that can fill 100,000 seat stadiums, but that's ok. When they do really well, the country tends to get behind them as an underdog that "stands for the right thing". Witness the stands in the 1996 Rose Bowl (mostly purple). We're a lot of big fanbase folks' "2nd fav". That works...until we beat them.