Another thing that the Schiphol analysis seems to forget is that not all markets have a lucrative passenger mix. AMS-MKE is a good example. Although MKE has significant economic activity, much of that activity is at most national and not global in scope, so there would likely be insufficient business cabin demand and at best mediocre economy cabin demand on the flight. So basically, if a carrier is not already in a market, there is likely a very good reason that has already been identified by a number of network planning departments at various carriers.
For what it's worth, within the presentation they cite reliance on is the Global World Cities research network, a long-running and well-respected organization from the University of Loughborough in England. Their methodology has consistently put Milwaukee as a tier 3 global city (Gamma) over the years including their 2020 release. Their methodology weighs the global interrelation levels heavily into the equation as opposed to simpler raw stats like population or GDP commonly used. This is not perfectly corelated to international air travel demand but it does seem to get much closer to that goal than flat measures like metro growth people like to cite here. Where skepticism for MKE is perhaps better rooted is that Milwaukee's de facto international airport has traditionally been O'Hare. The drive from O'Hare to Milwaukee is not substantially different than the drive from JFK to metro NY corporate powerhouses White Plains/Greenwich/Stamford/Norwalk. O'Hare (2019) had regular nonstop service to more than 50 international cities excluding Canada and Mexico, so the prospect of any single international flight retaining enough of MKE's international demand versus dozens of daily intercontinental nonstops in every direction from O'Hare is an uphill battle.
Here's the full list from their latest release: https://www.lboro.ac.uk/gawc/world2020t.html
And also their home page -- there's a whole lot of stuff there to give you a broader idea of the kinds of things they study which goes into the classification: https://www.lboro.ac.uk/gawc/index.html
From the US perspective:
Tier I (Alpha)
New York, Los Angeles, Chicago, San Francisco, Boston
Tier II (Beta)
Washington, Dallas, Miami, Houston, Atlanta, Denver, Philadelphia, Seattle, Tampa, Minneapolis, San Diego, Detroit, Austin
Tier III (Gammas)
San Jose, Charlotte, St Louis, Phoenix, Orlando, Baltimore, Nashville, Cleveland, Kansas City, Milwaukee
, Salt Lake City, Columbus, Sacramento
Tier IV (High Sufficiency)
Hartford, Raleigh, Indianapolis, San Antonio, Cincinnati
Tier V (Sufficient)
Pittsburgh, Portland, Las Vegas, Jacksonville, Richmond, Oklahoma City, Des Moines, Tulsa, Buffalo, Louisville, Rochester, Birmingham, New Orleans, Memphis, Omaha, Honolulu, Harrisburg