Investment income is flat taxed.
And inheritance taxes, which are very high in France.
If you want to increase taxes, consider taxing income more and capital gains at a progressive rate. Although I haven't seen good data on effects of say a 70% capital gain tax, might hurt th,e economy. I did some reading on this subject last month and the sweetspot was around 20% to 35% on that classification of income.
Do you want to take people's wealth and cap it? IE, nobody is allowed more than $5 million? What are you advocating for instead?
Would this actually cover France's deficit?
America's $6.75tn budget [1] would blow through our $140tn private wealth in 21 years. Even Norways $0.11tn budget [2] blows through its $1.6tn of private wealth in 14 years.
Perhaps the better metric is deficit as a fraction of private wealth? If that looks unsustainable, the problem is in publicly-held assets and services.
[1] https://fiscaldata.treasury.gov/americas-finance-guide/feder...
In the last 5 years French public debt has grown $750b [1]
Had that growth in wealth been taxed at the rate income was taxed (45%), that would have seen France's debt decrease - even with the covid mess.
[1] https://www.ceicdata.com/en/indicator/france/national-govern...