It was a Christmas miracle: Scaffolding at 409 Edgecombe Ave., a 109-year-old Harlem building once home to W.E.B. Du Bois and Thurgood Marshall, finally got taken down after 21 years.

The scaffolding had practically become a landmark itself.

And while it was the longest-standing scaffolding in the city, such “forever sheds” are all too common.

The average scaffold stays up 497 days — far longer than needed for any necessary repairs.

With no real penalty for leaving a “sidewalk shed” in place, cash-strapped owners hold off on removing them — some even doing so as a cheap fix instead of actually doing costly repair work.

No matter that the sky-blocking eyesores hurt businesses, boost crime and give the impression that the entire city is in disrepair.

In July, Mayor Adams announced his “Get Sheds Down” plan to speed up the removal of the sidewalk scourges.

At the time, the city counted nearly 9,000 actively permitted sheds (1,000 that had been up for more than three years) totaling about 400 miles of sidewalk.

Since the crackdown, the city says it has removed 500 sheds — or about 11 miles, a bit under 3% of the total linage — 75 of which had been up for more than five years.

At that rate, it’ll take 16 years to get them all down — except that more sheds will surely go up in the meantime.

Kudos to the mayor for making a real difference, but more drastic measures would be nice if the rest of the city’s political class would get on board with this basic fix to New Yorkers’ quality of life.