I did not claim secondary moderns provided no education. I wrote "secondary moderns provid[ed] a limited, constraining education to the masses." which I stand by. My parents taught in all three parts of the tripartite system, and were strong campaigners for comprehensive education mostly because of what they saw as the evils of the secondary moderns. They themselves had benefitted from the grammar schools (first generation graduates in the 1950s), but didn't see that benefitting one at the expense of five was ethical.
Your argument about "ah, it didn't constrain everyone" is a variation of the golden ticket argument (which abouteve seems to support for education, but might feel less comfortable with similar arguments for health care): yes, a few people were able to make it to university as mature students, but in an era of earlier marriage and child bearing, the number of those that were women was small. The transfers at 13 were difficult in practice, owing to syllabus differences, and transfer in at 16 is hopeless when the secondary moderns mostly stopped at 15. My mother fought to do O Level teaching at a secondary modern, which was hard enough.
The problem with the tripartite system is that assumed a static society. It assumed that other than the designated elite (and, pace Xenia, isn't it a co-incidence that everyone's designated correct cut-off for university is that of their own teenage years, to make sure that they are part of the elite?) everyone else would go into a factory or office at 15 or 16 and stay there. The elite were educated, so that they were flexible; the rest received training in typing and lathes. The problem is, that was still happening in comprehensives in the 70s and 80s, and means we have whole swathes of society that were simply not prepared for post-industrial economies.
I believe that knowledge is always better than ignorance, and education is always better than apathy. I don't feel the need to deny others education in order to make me feel better about my own, and I find the whole process of assuming either that education is only there because it's profitable (might not, for example, educated parents provide better support for their children's education, even if they themselves are not working?) or is a reward for passing some bar too Brave New World for my tastes.
Yes, there has been a reduction in some standards, particularly at A Level, but I'm not sure it matters. More people are being educated. Lives like that of my grandfather, blighted by having to work from an early age and thereafter a frustrated auto-didact, are now less common. Getting more people into education makes for a happier, more flexible society, and denying education to some to make it more valuable to others (the "too many graduates") argument strikes me as profoundly immoral.
But if people don't believe in the benefits of mass higher education, the question "how do you fund mass higher education?" makes no sense. So we talk past each other.
My father was told by a distinguished sugar chemist, in a department that had recently won a Nobel Prize and been at the heart of Tube Alloys, the British Atomic Bomb programme, that his undergraduate colleagues (this tirade was directed at one bench group of five people) were the worst year the university had ever seen. Standards were dropping because of the increase in numbers. Too many people were being admitted into the university, and there simply weren't enough bright people to benefit from it.
The year was 1955. University takeup was around 3%. Of the five people in question, two went on to become professors, one a very distinguished research chemist at ICI and the other two successful academic careers retiring as principal lecturers.
The sugar chemist had a building named after him at a London university. It's now a biology department (the Bourne Laboratory at Royal Holloway). Sic transit gloria mundi.