Tax avoidance and tax evasion are both highly sensitive to tax rates.  For more insight, readers might google Laffer Curve.
The exact threshold of tax rates at which tax revenues decline is a complex formula based in part on the degree of belief of the citizenry that they are receiving value for the tax that they pay.  Typically, tax rates over 50% are in the red zone, but it is important to note the additive impact of  income, consumption and asset taxes, as well as the odd head tax.  
It is pretty clear from this government's attitude on enforcement that it knows it is well  into the red zone, and each spending splurge and disbursement scandal just adds fuel to the fire that pushes taxpayers past the Laffer Curve peak.