Does anyone regret not following a formal curriculum?
No. Not yet, anyway. My older daughter is ten. For some things in life, it is useful to know how to take an exam, how to write a formal letter, etc. But you don't need years of full-time study to learn those skills; you can swot up on them fairly quickly when you have the need.
Many children (children who've not been put off learning by school) naturally start inclining to a more formal approach in some of what they do, sometime after the age of ten. My daughter is flirting with doing more formal studies. I'm not pushing it.
I remember arriving at high school after several years at a radical progressive primary school which required little formal work. The English teacher had quite a rant about our previous school: we might be "creative", she said, but we hadn't even been taught that one must leave a margin round an essay, put one's name at the top rather than the bottom of the page, summarise our arguments in the first paragraph, etc. I sat there quite astonished, thinking, "OK, now you have shown us exactly what you want us to do. It has taken you all of five minutes to do so. What's the big deal? Why did we need to arrive knowing these details already?"
I mean to say, I doubt that it's difficult to inject formal study skills into an older child or adult who has a passion for learning. But is it easy to do the reverse, to ignite a spark of enthusiasm in someone who has spent too many years concentrating on formulating exam answers which match the examiners' requirements?