The existing tax system recognises that individuals may have needs - education, health services, support when too old to work, support when not physically or mentally able to work, keeping people safe, etc etc. It also recognises that a flat rate of tax would mean that those on the lowest levels of income would not have enough to meet their basic needs so we have a system which says that if your income if way above what you need to survive you pay a bit more.

Discussion about unifying NIC and TAX into a single tax rate is nothing new.