I have a lot of time for the notion of scrapping N.I. and increasing income tax accordingly. The income tax rate could float in the initial years to cover any excess or loss. Maybe even raise it a bit higher and raise the threshold. The headline tax rate would highlight how much tax is paid though and would probably scare off any government.
I think it gets more complex. Remember NI is in effect ring-fenced to pay for contributory benefits (incl (New) State Pension). NICs do not go into general Government funds.
And the fund can (and often does) run surplus and then lends money to the Government. Government cannot just take the surplus (Social Security Administration Act 1992). ie Income tax does not go towards paying for state pensions, triple lock does not mean higher income tax rates.
So scrap it and a lot of ring-fencing is lost.
Personally I think if one wants to be a bit radical Universal Basic Income is maybe the way to go. Bold and some won't like it. A lot will depend on the levels and amounts as it means not only what you pay everybody (the UBI) but also the massive changes to taxation associated with that.
But it also simplifies so much eg no more pensions (which resolves those on state Pension being so much worse off than those on New State Pension and having to claim additional support, things like "triple lock" become irrelevant, much of DHSS and welfare goes, etc.
I can't pretend to have all the answers and there certainly would be challenges I don't know the answers to eg how to motivate people to get back into work when they can afford to live on their UBI and stay at home all day - but we already pay lots of clever experts lots of money to sort out those sorts of things.