They never existed.
I see this comment everywhere and it irks me. Here's my point-by-point, obsessed-fan's rebuttal:
1. It isn't a loft. It's an unusually large apartment.
2. It's explained on the show that it's a rent-controlled sublet illegally inherited from a grandmother of one of the characters. The illegality of the situation is a key plot point in one of the episodes.
3. It's obviously easier to shoot on a larger set than a smaller one. It isn't hard to suspend your disbelief on a fairly minor point. Would it have been a better show if they had shot it on a smaller set purely to satisfy people who want realism?
4. The size of the set is irrelevant to the storylines and characters. The characters don't otherwise live lavishly or appear to spend out of proportion to their income.
5. All but one of the main characters lives with a roommate or romantic partner throughout the entire show's run.
Cycle goes something like this. Factories in parts of the city shut down after it get cheaper to run them outside. Those spaces lay empty, rents drop, decay begins. Poor Artists move in. Area starts getting interesting. Yuppies move in cuz of the 'cool' factor. Property developers notice. They start buying everything up. Rents jump. Artists get kicked out. And you end up with boring rich people living in a formally cool area.
"Loft Living" is a great book, although it's been a minute since I read it. To quibble on a minor point -- if I remember correctly, a major part of that book was pushing back on the narrative on part of that cycle, that these factory spaces "lay empty".
Something she describes in this book is that these factory spaces were still very much in use by light manufacturers, and that it was the rich real estate interest specifically targetting this area for redevelopment that pushed a narrative that it "lay empty" so that they could more easily get the neighborhood rezoned and push out the manufacturers for more lucrative residential tenants.