The thing with the state pension was it wasn't put in as a means of people 'enjoying retirement'.
It was supposedly largely a safety net for those who'd lived a longer than average life but would most likely no longer be able to work. This was at a time when far more people had physical jobs that the body couldn't cope with in older age. At the time the average life expectancy wasn't much above 65 anyway so chances are even if you were alive you'd be in ill health.
It was also supposedly to keep employment moving. Making people retire opened up opportunities and thus you'd get a constant flow of people up the chain and the lower jobs could then be filled by the youngsters who in turn would later progress up the ladder in the same way. Nowadays it's a problem because as well as needing to provide more work for an increasing population in an era where many jobs can be automated and global wage competition, you also have to factor in the many people who're now staying in employment for longer. I think it's a good thing (those that I've seen continue working into old age seem to be healthier and happier than those who don't) but it does create problems for others.
Personally I think it was all a bit smoke and mirrors and the fact at the time was the biggest beneficiaries were the more affluent and in non-physical labour as they were the ones likely to live longer and be able to take advantage of it.
But if we take it along the 'safety net' principle then increases to 75 given the changes in life expectancy aren't that harsh. The issue, as has been pointed out, are that different types of work take different tolls on the body and thus 'retirement' will be necessary at different ages. Others would argue we have JSA and benefits for that if they're unable to work. Some would say they need to retrain into more sedate jobs (but for some people those types of jobs aren't suitable for them).
