So if the MA is ahistorical, why do we even remotely expect any of the planes and stuff to even look like WW2 planes in the first place?
Let's get the facts straight -- AH is ahistorical because there's no way to become fully historical in the first place. There's no such thing as an exact recreation so the developers try to pick one or two most representative features of the warring environment and use it to give the feel and aura to the game. However, there's no denying that a certain essential ethos of the game is purely dedicated to the historical WW2, and if it is deemed possible, being realistic and historic is ALWAYS preferred over being unrealistic, 'gamey', or 'dweeby'.
It's a simple thing.
Flying with something like 50% + DT is basically, unreconcilably dweeby and gamey. No matter what kind of wording you use to describe it, it's just as gamey stuff as Lancasters deckbombing at 1k alt and spraying bombs all over the place, or the insta-AA bombers on the ground. Sure, both cases may be technically possible, but they are also situationally very very improbable.
Range and performance is mutually incompatible. It's simple logic. If you want to fly for a long time, you take a heavy fuel load. The pros are the flight time, the cons are the weight. If you want the best performance, you take a light load. The pros is the performance edge, the cons is that your flight time is limited, so you must find a fight quickly, and a lot of climbouts is not optional.
However, a configuration like 50% + DTs is an exploit of a real life "possible, but very improbable" situation which has quickly become the norm in the MA. Basically, it twists the basic logic - a plane can fly long and high, and despite that it can always instantly revert to a fully ideal, low-fuel condition at the touch of a button.
A plane should not carry needless amount of fuel. If you don't want a heavy P-51, then you should up with 50% fuel in the first place, like everyone else. The flight time will be considerably shorter and your range is limited - but that's hardly surprising. Flying with half the normal fuel load is SUPPOSED to be like that. If you want to fly longer, like a true P-51, then up with 100% or 100%+DT fuel load and fly longer, but suffer the problems of weight - like everyone else. People are SUPPOSED to be heavy when they want to fly long distances.
That's basically it.
Making DTs availale only after 100% isn't taking away anything. It's just making the planes that loved such a config, like every other plane. Being able to fly a long distance is already an advantage by itself. Why should such gamey use of DTs give another tactical advantage to long-legged planes - the ability to fly a long distance and still be able to fight at light weight?
It's not penalizing anything. If there's anyone being penalized, it's the short-legged plane users. Not only do they already have a disadvantage of having a shorter flight time, but they also have another disadvantage in that they can't choose to instantly dump fuel to make the plane 50%+ lighter than it initially was, on a whim.