The problem with this philosophy is simple: there's no guarantee that you would have been ready to learn that then. I'll use my own learning techniques as an example.
What I wish I'd knownI took French and Italian as high school subjects, and didn't learn all that much in either, considering the time cost. When I started learning Spanish and Gaelic, I was simultaneously studying English language at degree level with the Open University. When I moved on to degree-level Spanish, I was annoyed that none of the linguistic concepts I'd been taught in the (mandatory!) English module were used to speed up the learning process. Too many things were left undescribed and confusing to my fellow students, when a ready explanation was available.
The English course didn't just occupy itself with the traditional grammatical concepts of parts-of-speech and syntax, but addressed language in terms of multipleframeworks: Halliday's systemic functional linguistics and metafunctions, the idiom principle vs the open-choice principle (language as a series of set phrases vs a series of free grammatical choices), lexicogrammar, corpus linguistics and collocation, directness and indirectness, agency and affectedness... the list goes on.
Knowing each of these concepts allowed me to disambiguate or disentangle my own confusion about new word or phrase forms, and I know that I would not be as successful a language learner today without having undergone that course of study. (This of course is in direct contradiction to a significant number of language learners who flatly refuse to accept that conscious study has any value whatsoever.) Now I have often wished I had learnt all that earlier... but would I have been ready? If I had not already learnt (a little of) two foreign languages, would I have been as open to the instruction I was given? Or if I had not been actively engaged in learning two languages simultaneous to that study?
I cannot know, therefore I cannot say for sure, but I believe that it would have helped. I temper that with the knowledge that the course I took would be overkill for most learners, but that there are certain concepts that help immensely. (eg indirectness as a form of politeness; compare; "shut the door","can you shut the door" and "could you shut the door" -- the imperative is direct and rude, can is somewhat indirect, and could is very indirect, making it polite. This same rule about indirectness holds for most European languages, but not e.g. Japanese. It is very easy to draw a learner's attention to the presence or absence of this pattern, but many courses fail to even attempt this.)
So I would advise learners to learn this stuff (if I only knew of a good book!) but I would also caution them about trusting my advice blindly.
What I might have wished I'd doneWhen I bought myself a DVD player, it was because of language. I had taken a French level test with the OU before starting my study, and while my reading and writing were at a pretty good level, my listening level was disproportionately low (incidentally, this worked in my favour in the end, as otherwise I might have started with French rather than English) so I needed to improve my ear.
Fast forward 9 years, and I now find myself watching Gaelic TV or French films and noticing unusual turns-of-phrase on the fly, by comparing the soundtrack and the subtitles. This, as it turns out, is a highly effective way of learning the sort of stuff that doesn't make it into the textbooks.
So would I recommend it? Not really. 9 years is a long time, and it has taken me a lot of practice to get to the point where I can do this -- when I started, the subtitles would soak up my attention, and I wouldn't just "not understand" the audio track, I wouldn't hear it -- my mind blocked it out. It took me several years to stop blocking it out, and years after that to be able to consciously follow both languages, and even now I'll generally slip into either reading or listening -- it takes active concentration to do otherwise.
Now I could have forgotten all the time and effort and told myself that I obviously hadn't worked hard enough soon enough, and that if I had, I would have learned even better and quicker than I did, but I simply don't believe that's true. I don't think this technique, which is now one of the most useful in my arsenal, is actually worth the effort of learning to most people.
This serves as a useful reminder to me that everything I do now is a refinement and improvement of things I've been doing for years. These techniques can't just be "done", they have to be learned; which means that I can't just "tell" people to do them, I have to teach them... or keep my mouth shut.
So there we have another problem in learning from successful learners: all too often they will simply tell you what they do now, with no real conscious understanding of how hard it was for them to reach the point where they could.