I would argue it really isn't. Massachusetts still has some of the strongest union protections, is one of the most unionized states, and Baltimore has worked hard to maintain an incompetent government for decades. Given the outrageous rents in nearby states and cities Baltimore should have kept growing but has instead shrunk to a half million.
The rust belt as a whole was primarily just companies escaping unions. Cheaper labor may have been an incentive but it fundamentally started with companies who were forced to move production, and US regulation which prevented them from benefiting from local supply chains once they became unionized.
I'm far from a union sympathizer, but you've already moved from your initial, absurdly simplistic description of the issue to a major symptom (decades of ineffective government, ranging from benign incompetence to intentional malice).
Also, the rents in nearby areas to the north are far from outrageous (hint: there are no major cities for a couple hours), and the Baltimore metro area continues to grow because the area is desirable, even though the city is completely mismanaged.
It's poorly managed, but not due to over-regulation IMO as the other commenter implied (not to say that there aren't some areas where deregulation could improve things).
The core issue today is that the population of Baltimore consumes significantly more resources than they produce because the city managed to drive away a significant portion of the middle class and the government is so inefficent, leaving too many people who are unable to move, are actively part of the problem, or are one of the few that is so wealthy that they are largely unaffected by the happenings around them.
How it got there is a much more complex story than "regulation and unions" but is to some extent irrelevant if they want to look forward to solve the major systemic issues that exist.
If they want the city to recover, they need to provide the basic social services that most people want: clean streets, physical safety, acceptable schools, and minimal interference in everyday life. That has not been a priority of any city government for decades, though I think the current mayor is trying.
If poor economic choices, inept management, and inefficient government don't qualify as “badly regulated”, then you are using a stricter definition of regulation than we are.
The original commenter clearly was stating that overregulation was the issue. You're the one who keeps bringing up that it's "badly regulated", which is both an odd term and not what was originally stated.
The rust belt as a whole was primarily just companies escaping unions. Cheaper labor may have been an incentive but it fundamentally started with companies who were forced to move production, and US regulation which prevented them from benefiting from local supply chains once they became unionized.