Yes, I'm pretty suspicious about the Conservative plan to remove NI.
You could argue that by just folding NI into income tax (dropping NI and increasing income tax to match) it's broadly the same effect and simplifying taxation brings efficiency benefits.
But it's definitely a useful lever currently. Reducing NI is a way of reducing the tax disincentives to work whilst not reducing tax on unearned income. That makes it a slightly cheaper way to encourage work participation than simply using Income Tax. So that's a disadvantage of amalgamating them - you lose that targeted lever.
And you'd have to cover not only the 8% employee contribution but also the 13.8% employers contribution in order for the government to get the same amount of money. You'd be assuming that salaries would increase to cover that 13.8% now coming from employees gross pay instead of from employers directly (that would leave everyone with the same amount of money after PAYE) but it's unlikely to work perfectly and the headline tax rates would look scary!
Eg if you currently earn £35k
You currently pay £4486 income tax and £1794 NI. And your employer pays £2578 NI on your behalf which you never see.
So the employer pays £37578
The government gets £8858
You get £28720
So fold that into Income tax but keep everyone paying/getting the same:
You'd get a 7.3% pay rise so that your gross salary is now £37578 (what the employer used to pay with their employer NI contribution added)
You need to give the government £8858 on that salary, so your standard tax rate above £12570 would now be 35%.
It ends up everyone paying/getting the same amount, but can you imagine a government saying they intend to raise the standard tax rate to 35%?!
So why are they doing it?
NI was introduced - and is still talked about - as a way of funding the state pension as well as out-of-work benefits. I think that's very significant.
If it was removed, it would make it much easier for the government to remove the state pension in say 30 years time, when the people reaching pension age haven't paid NI all their lives. Since then they won't be able to argue as we can now that NI gave us good reason to believe the promise that we'll get a pension.