Trying to find something about when non-rhotacism developed in Barbara M. H. Strang’s A History of English (a great book with an inadequate index), I stumbled on this passage, which I provide as a public service:
The sound /ŋ/ now appears medially and finally in stressed and unstressed syllables, as in singing; it has never been accepted in initial position. Its extension to unstressed syllables is quite recent, and has spread from middle class into general usage under the influence of spelling (or so the expression ‘dropping the g’, for the older pronunciation, indicates). As recently as 1936 Wyld retained his 1920 comment that the older pronunciation (/ɪn/, /ən/) was ‘still widespread among large classes of the best speakers, no less than among the worst’ (op. cit, 283). He describes these forms as ‘of considerable antiquity’ and ‘at one time apparently almost universal in every type of English speech’, he notes that Swift had objected to them in the early 18c, and in 1801 Walker ambiguously remarks that the best speakers use ‘g-less’ forms, but yet these forms savour of vulgarity (ib., 289). During the same period unease about the pronunciation was shown by hyper-correct ‘reverse forms’ in –ing where it had no place historically – as in lupin, chicken, children. The movement towards –ing gained momentum in the 19c:
Apparently in the twenties of the last century a strong reaction which set in in favour of the more ‘correct’ pronunciation, as it was considered, and was in reality an innovation, based upon the spelling, was so far successful that the [ŋ] pronunciation . . . has now a vogue among the educated at least as wide as the more conservative one with –n (Wyld, loc. cit.)
Let me just repeat the money quote: “the more ‘correct’ pronunciation, as it was considered, … was in reality an innovation, based upon the spelling.” Those people who say “I’m goin”? They’re historically correct. The people who laugh at them for “dropping the g”? They’re historically wrong, wrong, wrong, and should be pointing the finger at themselves for abetting the degeneration of our precious mother tongue.
This has been #3514 in the series “For Pete’s sake, stop worryin’ so much about what you think your neighbor is saying ‘wrong.'”