Last week’s near-tragic collapse of a boiler ventilation shaft at a Bronx NYCHA building is emblematic of the decay of the city’s public-housing stock.

The collapse of the 20-story chimney rained bricks down onto the sidewalk and left a gaping hole in the façade of the Mitchel Houses high-rise — and another hole in Zohran Mamdani’s agenda.

The city’s embattled public-housing agency faces a $78 billion repair bill for new roofs, pipes and boilers at its 177 crumbling complexes as it begins another heating season with constant furnace and boiler issues.

The only way forward is the route opened by President Barack Obama: Privatization of public-housing management to allow the necessary repairs and prevent yet more of the horribly-deferred maintenance that’s left so many NYCHA buildings is awful shape.

The city made progress in that front in the de Blasio years; the Adams administration’s latest initiative is a public-private partnership to transform Manhattan’s dilapidated Fulton & Elliott-Chelsea Housing projects and the surrounding neighborhood. 

But Mamdani rejects any private-sector role: He wants to sell $100 billion in municipal bonds (including $30 billion already budgeted) to add 200,000 new units of public housing — more than we have now! — and double city investment in preserving existing projects.

He’d need Albany’s OK for that extra $70 billion in bond sales, which would add about $5 billion a year to the city debt-service costs, pushing the total to more than a fifth of the annual budget.

Which would surely raise rating-agency alarm bells, and trigger a drop in the city’s bond rating — forcing it to pay higher interest rates and sending debt-service costs up even more.

Public housing, in short, is one more front where Mamdani’s socialist ambitions greatly exceed what the city can possibly afford.

Don’t ask which promises he’ll wind up choosing to keep; ask whether he’ll be able to deliver on any of them.