Doesn't that high number presume that businesses won't move over the WI and IN border to avoid the tax increase? Most of the economy of IL is minutes from those borders.
Those borders aren't far, but it's not like Kansas City. No one should be worried about tens of thousands of people's jobs being relocated from central/suburban Chicago to rural Wisconsin. People live in a place like Chicago for a whole host of reasons beyond just 'the job is there' and we need to forget this lowest-common-denominator argument.
It does. I'm sure some would move, but New York and California maintain high tax rates without turning into ghost towns. To be clear, I'm not arguing the union position is correct, just that it's not ridiculous on its face. It's feasibility depends on subtle second order effects like the one you describe, not on raising tax collection to obviously impossible levels.
NYC is sui generis. California is geographically hard to flee from --- but people do flee SoCal for Vegas.