Density is the key word...!
A 5 to 6 levels old buildings in the Marais district of Paris are three time (at least ) more dense (ratio of built m2 per parcel of land m2 to which you attribute a given ratio of people per build m2) then the worse "banlieue" with  high rise towers and very long buildings...!

The latest is considered more Hygienistic (sun, space, gardens)! But today the highest cost per m2 is in the Marais !

Speaking of Paris and all those homeless people... If SNCF and RATP (train and metro/bus) would give up the land they don't use today... Almost one quarter of Paris could be built for people.
If we covered only half of the Périphérique (the ring road around Paris) we could add 1/6th to Paris again...
Then at last if we added two to three levels to the old Hausmann buildings (easy today), we wouldn't even need most of the large suburbia around Paris...!

It's (as always) a planning matter... With most decision maker in politics that changes every 4 years... You cannot project very far ahead in time! With most of these territorial responsibilities left over to mayors, you just can't do it ! As it can't be local... Anymore.

"What can I do, What can I write, Against the fall of Night". A.E. Housman