I think the problem we have starts with hyperinsulinemia before we get to the resistant stage, and this is the element that is part of us. The insulin resistance, like the type 2 diabetes may improve over time since muscles and other tissue are always being renewed and it is the tissue that becomes resistant.
I have had some success in reintroducing carbs on maintenance, (just over 9 months in and at target again today after a couple of months a few pounds above) but in nowhere near the quantity I used to eat and I am still an almost complete junk food free zone. White carbs and too sugary food wake the carb monster which is the mental manifestation of this hyperinsulinemia as far as I can tell. Your body floods with far more insulin than it needs to deal with the food you have eaten, and this prompts your ‘second brain’ to shout for more sustenance. I think this is also the mechanism behind the effect of sweeteners, raising both insulin and cravings, as the body expects more sugar than it gets.
Whether you remain at keto levels of carb or not this is a super healthy way of eating, and I agree that as long as you stick to generally this way of eating going forwards then the gradual increase of calories up to and even beyond your calculated TDEE is the key to keeping your metabolism fast.
With the Biggest Losers, I wonder how much guidance they are given on the transition back to a less regimented existence? Are they advised to reduce portion sizes to match the lower physical requirements? This would tend to keep the metabolism slower. I have been using the ‘reverse dieting’ transition where your body can adapt to the bigger portion sizes naturally. I have a couple of times done this gradual increase once from 800 calories and once from 1250 calories at 50 calories extra a day and in both cases only stopped when I got to 2500 calories a day, still not putting any weight on, although the earlier losses while still increasing the calories had stopped. Reading about reverse dieting, someone as an experiment carried on up to 5,000 calories and his metabolism just carried on speeding up with no weight gain. We could only manage that by keeping carbs low as the insulin over production would just use excess carbs as a chance to store more fat.
I think if you transition properly, and eat a varied diet with little or no junk food, there is no reason why your metabolism can’t be just as fast as it ever was, maybe faster if you have built up more muscle.