Recently the city of Toronto surpassed Chicago in population. This was done the usual way, through annexation.
So what happened? Did two million people move to Toronto? No. Toronto
moved to them. In 1998, the old city, which occupied the lakefront and
the business district, amalgamated with five suburbs to form a
Chicago's prospects for annexation, contra this article, are dim. The two most promising candidates are Oak Park and Evanston. Both cities are on the border of Chicago and have lines of the L running through the center of town. Their infrastructure and economies are tied into the core city so intrinsically that they might as well already be part of Chicago.
But they won't be annexed. I mean why would they? Annexation means losing political autonomy to a corrupt political core, diluting the local vote into the big city, and diminishing municipal services. Oak Park and Evanston have far superior schools, police, and roads compared to Chicago. Why should they ever consent to annexation? It won't reduce their taxes or increase their city services. Until Chicago can offer their suburbs something better the city borders will be ossified and the city will not grow.
Most cities grow through annexation. Even Manhattan is not as dense as it was about a century ago. Core cities over the last century reduced density. Even the core of Paris is less dense than it was a century ago. This trend shows no sign of reversal. So in order to prevent a long term decline, Chicago needs to get its core city services in order. That means using its economy of scale to provide superior police, fire, schools, and roads at a cheaper price than the surrounding suburbs. But until the city makes real reforms in crushing the high labor costs and cronyism the city will continue to atrophy.