I assume this must be some building code law, otherwise there's a lot of room to get creative with active ventilation. If assumptions were challenged, there's a lot more possibilities for how to convert these spaces. A lot of people would be willing to rent a windowless space with a shared bathroom (e.g. https://www.fastcompany.com/90740511/heres-what-its-like-liv...) if it meant they could live in a great location. Example: What if windowless apartments had a built-in 12" video wall like the one this YouTuber inexpensively DIY'ed: https://www.youtube.com/watch?v=bv88wM6AN6Y