The introduction of a flat tax rate, as opposed to progressive tax bands, would be required to meet two simple criteria to be viable.
- Equality of Government Income
- Fairness of Tax Burden Changes
Number 1 is a no brainer. The Government still needs all the income it currently receives. Even if flat tax collection was cheaper than the current situation, a dubious assumption, the cost of tax collection is minor against the amount of tax collected.
Number 2 is where any real problems lie, to be seen as any way a legitimate change the impact of a flat rate cannot be simply seen as a give-away to any section of society. The only practical way to achieve this is to modify everyone's current income (at least pay rather than investment income) based on the changes in tax. After all, everyone's pay is currently based on the idea that they'll lose a certain amount to taxation, if tax changes dramatically it is only logical that pay would change dramatically too. The work required to do this and the compliance cost to ensure it is done fairly is significant to say the least.