Merging NI and income tax is logical in theory, but it opens lots of cans of worms.
Various benefits (both State Pension, and working-age benefits) are based on NI contributions. If the combined income tax + NI applies to pensions, then they will be taxed twice on the NI element (since the original contributions only got relief for income tax). That's just off the top of my head — there are probably other issues.
There are probably ways to deal with all these issues. But it's not easy, and could upset plenty of people (some of whom are Tories ). So I wouldn't hold my breath.
It's been investigated at various times and always put into back into the "too haaaaard" box. One huge issue would be NI credits, too. Another one is the grey vote - remember, people over state pension age don't pay NI. It would take a very brave Government to lumber them with a 12%ish additional tax. You'd also have to have special provisions for the transition from the old to the new system, and these would probably have to last for decades (until the last NI payer has died). Although this could probably all be solved somehow, it seems to me that it would be a massive, confusing, rather costly, change for not much benefit.
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It was answered in post no. 2 in this very thread.