THE EXISTING SYSTEM!!! How many times?????
People get credits towards state benefits such as the state pension. They DON'T need to pay any NIC to get those credits! What's so difficult to understand???
The existing system is currently applicable to the unemployed, disabled, parents of young children, part time workers earning between the two lower NIC thresholds and self employed earning between the two lower NIC thresholds.
The existing system can be extended to include others such as those with buy to let income, large amounts of interest/dividends, foreign income, etc., via the self assessment system as such people will already be submitting SA returns and can gain "credits" for state benefits just like self employed people gain credits via the SA return.
For workers, it's similarly easy - those whose earnings are between the lower NIC thresholds gain credits via the payroll submission system - without paying any NIC - that system can be extended so that workers on higher wages can also get credits for state benefits without paying NIC, same as with self employed on higher incomes.
I really don't understand what you're not getting here. It's blatantly obvious that you don't understand how NIC credits are given for several groups of taxpayers without them paying any NIC. Just extend the existing system to everyone. You've then got a system for granting credits towards state benefits such as state pension without anyone paying any NIC. Probably at least half the population have already gained some such credits over their lifetime via the existing systems.