That certainly sounds like a success to me - very well done! Your current numbers are pretty much perfect and if you're managing to achieve those with modifications to diet, supplemented by Metformin, I can't see why a doctor would want to change anything.
Did the doc just say that to scare you or something, as insulin for a T2 is only really considered when everything else has failed to work for them. I'm on insulin, but I managed with diet alone for over 18 years, then only deteriorated after an unrelated illness than I didn't bounce back from as well as I'd been before. I battled with assorted ineffective meds for a further 4 years or so before I succumbed and took up their suggestion to start insulin and it turned out to be great for me (at least for a while, it's gone a bit odd lately).
But I've never heard of a doc suggesting insulin to a T2early on - unless he thought that you might be a late onset T1, but that wouldn't be easily fixed with diet. Even at 12/14 you wouldn't initially consider insulin, I was in the upper 20s - after a long period of individual effort and max doses of meds - before I started with it.
Well done, that sounds like fabulous progress, keep up the good work!