I don't think it's really fair to just fold NI into tax, but then say that everyone from that point onwards - despite paying the exact same total tax - doesn't get a pension.
I suppose it would be fairer if they actually removed NI, and then tapered the state pension recieved in retirement depending on how many years NI had been paid. Ie someone with 35 years NI contributions gets the full state pension, someone with 12 years NI contributions (and they don't pay any more once it stops - but also don't get the stamp) will only get 1/3 state pension.
I still think that would be a spectacularly bad idea though, since there would inevitably still be significant means tested payments, which makes a huge disincentive to saving.
And as a pp said, anyone who has enough pension for an income of about £60k already effectively gets no state pension, since they pay as much tax as the state pension is. 'No state pension' will fall at £54k if Labour add NI payments for pensioners.
That's not far off the £45k for a single pensioner / £62k for a couple which is considered necessary for a 'comfortable' retirement- especially if one member of a couple has a much bigger pension than the other.
And that will get even worse, as frozen tax thresholds continue to bite.
We're not talking millionaires here. Someone with just enough money for a 'comfortable' retirement already effectively isn't given any money by the state, they just get let off paying some income tax. 
No additional means testing is needed.