I struggled with issues like this. Why build things of mostly grandeur when there is so much suffering in the world?
Maybe this is just a rationalization on my part, but I think we need things of beauty in our public spaces — for everyone to enjoy. The Notre-Dame fits this bill very well.
Enjoying these scenes is part of human nature. Imagine the life of a French peasant 500 years ago. Chances are that your house was not a work of inspiration. But for a few hours a week, one could attend Mass at cathedral and partake in something that was inspiring to the soul.
If the billionaires were not footing the bill, the French taxpayer would.