Knowing Paris as it is now and comparing to the photographs in the article I am happy that cleanup was performed. The old architecture was maybe interesting to some extent, but very close to a huge slum.
Note that you can also clean things up without tearing it down entirely. Many cities have retained historical centers in that style without keeping the "slum" bits, and it does makes for more interesting walking around than huge boulevards and avenues.
There's an architectural movement to come back to these more human-sized city designs too: smaller streets, more mixed use, less emphasis on large (and motorised) transportations, incorporation of structural shading and breeze-shaping, ...
It works for small towns, not capital cities with multi-million inhabitants. Even with the (fabulous) dense underground network you need larger boulevards and parks, not 3 meter wide streets everywhere. I am not a fan of Champs Elysee (there are so many better places), but it gives some air to the city. I think Paris 200 years ago was claustrophobic and a maze.
Maybe. But had those buildings survived they would now be attractive, clean, and very expensive.
From the perspective of today the problem was that the families living in the small apartments was way too big and did not pay enough rent to maintain the buildings. They just did not have enough rich single professionals willing to pay chic apartments and studios back then.
Or student with rich parents seeking a studio downtown.
The 2-300 year old buildings would be easier to raze and rebuild than to put electricity and plumbing in it, improve resistance to earthquakes, make it safe to live in case of fire or other events that requires evacuation. They were not safe on modern standards.
There's an architectural movement to come back to these more human-sized city designs too: smaller streets, more mixed use, less emphasis on large (and motorised) transportations, incorporation of structural shading and breeze-shaping, ...