NI banks are very likely to continue to issue their own notes for as long as they're allowed to.
It's a very nice little earner for them.
It's like printing free money for the bank.
If you read what's written on the NI notes it says
"Bank X promise to pay the bearer on demand Y pounds sterling"
They don't have to keep matching sterling for every northern funny money note they issue.
This also explains why people turn their noses up at them on the Mainland and Down South as the issuing bank isn't as safe as the Bank of England.
I've got 20 of them myself that I can't get rid of. The UB machine Up North issued them to me when I would have much preferred the Queen's currency