The problem with this view is that there are instances where the 'canonical' pronunciation in one group is the 'noncanonical' pronunciation in another, and vice versa. For example, in British English, 'r' deletion is part of the 'canonical' pronunciation and it's only nonstandard dialects that are rhotic. In American English it's the other way round. So in general, it can't be that features of the pronunciation itself somehow determine whether or not it is canonical.
I doubt you really know what accent you spoke with when you were three or four. As to why you picked up an accent somewhere close to SAE, it's most likely for reasons of prestige.
I don't have to remember. I have recordings. Things like 'r' are a bit of a puzzle. What determines when a letter is silent? I don't actually believe pronunciation is absolute or fixed in time. But I don't believe its perfectly relative, either, or that "standard" does not have a meaning. Most accents sound like vowel sounds aliasing as other vowel sounds. I think the standard accent just doesn't alias nearly as much among all the vowel sounds, and I think that's meaningful.
>What determines when a letter is silent? I don't actually believe pronunciation is absolute or fixed in time
Not sure what you mean here. There are rhotic dialects which lack a rule of 'r' deletion and non-rhotic dialects which don't. It has nothing to do with letters. It has to do with the presence or absence of a particular phonological rule. The relevant feature most certainly is fixed in time, in the sense that speakers of non-rhotic dialects consistently delete /r/ phonemes following a vowel whereas speakers of rhotic dialects do not.
>I think the standard accent just doesn't alias nearly as much among all the vowel sounds, and I think that's meaningful.
This is simply false. There are non-standard dialects that make more distinctions between vowels than standard dialects.
I doubt you really know what accent you spoke with when you were three or four. As to why you picked up an accent somewhere close to SAE, it's most likely for reasons of prestige.