Sorry Minato, your (always insightful) pictures make it too hard for me to direct quote.
Essentially, it is this:
a) Paris' core is a landmark as a whole. It's not just the beauty of the buildings, but their significance. Post-enlightenment Paris was the centre of the world, the point where the spotlight was focused on Paris. Any modern buildings will not have the same significance, even in the future, as Paris takes second place to other global Alpha cities (it is an Alpha + city, versus an Alpha ++ like London).
b) Tour Montparnasse thankfully can only be seen looking down a few streets, like Rue de Rennes. More towers would obstruct more sight lines and impede on more of the atmosphere, as, from an architectural standpoint, France has not been at the top of aesthetics. Even if one could design a fairly nice skyscraper, such as the fairly pretty hotel near Palais de Congress, chances are it would age comparatively very poorly.
c) There's no reason why construction couldn't be had outside the Periphique. As most of the population lives there, and land is cheaper, it would be better suited to the business practice.
Yes, I can see both Minato and Shocker's points. There is a fine line between preserving what is important and meaningful, and advancing the city. I do not want another Bruges, for example. That's why I'm not against modern infill, or even modern redevelopment (with nice architecture). What I am against is revising the planning and massing, which reflect the last time Paris was the seat of the world, by building ostentatious skyscrapers outside La Defense. The city, like Rome and the other lucky-to-be-spared European capitals, should try and preserve its soul until a new generation brings it back to the forefront of civilization.