Obviously this is an idea that should go ahead.
But there’s two important provisos.
1) What’s going to happen to investment and pensioner income given that those don’t currently pay NI? For the logical reason that NI is supposed to pay for the work related elements of the welfare state (unemployment pay, pensions etc). Simply combining NI with income tax would raise the tax rate upon capital income by nearly 25 percentage points. That’s probably not a good idea.
2) It’s vital that employers’ NI be wrapped in as well as employees’. For it’s clearly and obviously a tax that falls on the workers. So the workers should be able to see that it does.
Of course, the major point of doing this is to explode, once and for all, the idea that we have low income tax rates in this country. And given that everyone will see the burden they’re already paying make it more difficult for tax and spend to be a popular programme.