Is postponement of State Pension Age inevitable
Yes
But this is missing the point that our economic development has been enormous and unimaginable since those days.
Only matched (if not exceeded) by (a) increase in life expectancy and (b) general population growth
I remember the talk of a leisure society on the back of the huge technological advances.
On "Tomorrow's World". But other than futurists, it has never been a topic for more immediate planning becasue it was never on the near horizon
So an argument can be made that society can afford not only to maintain the 65 retirement age but even to reduce it without adversely affecting the standard of living of those at working age.
Not sure the meanderings of Tomorrow's World or similar count as "the argument being made". There has been a pension concern knocking around since I did Leaving Certificate Economics - late 80's. So there is no question of "reduc(ing) it without adversely affecting the standard of living".
Of course even though society as a whole could afford longer retirement...
You haven't made that case...
...a situation where there is 2 workers for 1 pensioner could give rise to demands by the workforce for a bigger slice of the cake even if they are themselves enjoying more cake than ever due to economic growth.
A 2:1 ratio won't change the demands for the tax payers looking for a slice of the cake. It'll be the same as now. The difference is that we currently have 5:1 and therefore there is some scope to meet tax payers demands. Therefore a 2:1 ration means that there will be significantly increased transfer from tax payers along with a significant reduction in "the cake" and a reduction in their "slice"