That's quite easy: billionaires are not contributing fairly and constantly evade taxation, thus concentrating more and more of the national added value.
Where is your evidence that they're "giving all the tax money to billionaires", comrade?
So once you've milked the 43 French billionaires to extend French spending by 2.5 years or so (this assumes you extract 100% of their wealth into actual liquidity), what's your plan for year 3?
You could kill them all and take every penny they own and it still wouldn't solve the problem.