NI is just a tax. It is loosely related to pension rights. It does not build up an NHS entitlement, a right which applies to all residents in the UK: as the NHS budget is met from general taxation.
There are millions of old people paying tax in the UK who could not afford the choice of moving to Spain for a 'better' cheaper life style - I have some sympathy with those at the margin who are caught but not a lot I'm afraid.
Sorry but that's a poor response. For NI also include income tax. Pretty much everyone I know out here didn't work out here but worked in the UK, choosing to retire out here. And some of us, due to who our employers were at sometime in our working life, still pay UK income tax. Your post also makes no mention of the reciprocity offered by the Spanish govt, i.e. to maintain the status quo. Perhaps all those Army, Navy, Air Force, Police, NHS, Fire Service, Civil Servants etc who are retired abroad but who are paying UK income tax should tell HMRC to get stuffed - its not a small amount. If we pay income tax in Spain we can access the Spanish health service but we have no choice where our govt sponsored pensions are taxed, hence no access the the Spanish health service without paying extra.