... There's certainly an argument to be made that we should just roll it ( National Insurance ) into income tax and be done with it rather than having a separate line item. NI would then just be a credit on your record. Effectively meet criteria x gain one years credit which is close to the current system just without any taxation element.
You are ignoring the fact that NI contributions both employer and employee are based on earnings while personal tax is based on income from all sources including investments & pension.
For those who are not employed but have non-earned income beyond the 20 - 40% tax threshold the 20% increase is significant. For those who are an employee the effective increase is only 10% because NI contributions drop from 13.2% to 3.2%.
I unsure now for the self-employed but I thought unlike years ago their combined NI and tax contributions based on gross trading profit ( i.e. total payments received less cost of sales i.e. items bought to sell / materials used e.g. builder and allowed expenses ) was similar to an employee.
For the employer as I interpret the
government website page listing the thresholds they pay 15.05% on all of the employee earnings over £175 per week / £758 per month.