What are you saying exactly? How much should spending have increased to account for inflation, population size, and population age distribution? Please give a specific number. Would that be a good use of limited resources?
Obviously NHS funding can't continue growing faster than inflation forever. That is unsustainable. At some point the NHS would consume the entire government budget.
And also, the population distribution can't continue skewing older indefinitely, that's unsustainable. At some point geriatrics would make up the entire population.
And yet, both of these trends can continue for quite some time, and one trend can outweigh the other for quite some time.