Vietnam is where the bombing campaigns such as Arclight and Linebacker drove the Vietcong underground or into neighboring "off limits" nations.
The Chu Chi tunnels proved to be a very effective strategy for the Viet Cong, one which the Americans never adapted to. In spite of the incredible quantity of bombs dropped on the "Ho Chi Minh" trail, ultimately the Americans were not able to disrupt it. All we really did was shell shock the Cambodians so bad we contributed to the takeover of a cartoon like villain in the Khmer Rouge.
I understand the attempt to try and understand the logic behind the bombing campaigns of the war, but it was a war that had one foot planted in the old method of warfare: bombs and artillery.
A great many lessons were learned and several chapters of old traditions were closed by the time the war came to a close.
One of the main reasons I bring this up is that the lessons really were not learned, or were learned very slowly, because we doubled down on the terror bombing in Korea (especially) and heavy bombing still in Vietnam, even though it ultimately wasn't very effective. Gradually we have shifted, with the help of technology, much more to precision bombing, but there is stil a mentality in some circles that "mud moving" and "more tons of TNT dropped= more success" and worst of all, that killing civilians is winning war. The legacy of Total War, in other words, still exists in the air war, and the use of weapons like Scud type ballistic missiles. In our nuclear arsenals.
In the case of bombers, every belligerent nation possessed them and every nation used them on cities during the course of their campaigns.
It just happens that the Allies had more but don't think for a moment that if the Axis had the same quantity, that they wouldn't hesitate to do the same.
The "Amerika bomber" program was intended to have a bomber capable of flying from France to New York City and bomb it. They weren't looking for a type that could fly in and only bomb a specific place - the city was the target.
Needless to say, I have no doubt about that whatsoever. The Nazis had little concept of moderation in war, neither did the Japanese.
Sending in Mossies, P-47s, Typhoons and the like is a noble idea, but the AA would have eaten them alive - the Germans would have adapted. As it was, their 88 and 120 batteries scourged the heavy bombers at altitude.
Well, that is the real question isn't it. I think this is where the argument really is and where you have your best point. AAA was always a problem, and so were German fighters almost to the end, however I don't think the Germans ever really adapted to the Mosquito well enough to blunt it's effectiveness, and the Allies too could adapt.
I do think there was a role for heavy bombers mind you. I assume most here saw the footage of the Grand Slam bomb detonated by the Polish recently? It gives you a sense of the mighty power of the raids which were going on against German ships at that time. I still think dive bombers were a more effective to get them but dive bombers were a major design challenge which often 'broke' otherwise promising designs.
However, I grant you, low altitude strikes are dangerous, even today ground fire and AAA are a problem. I think that would be an interesting scenario, but I admit I can't be certain which way it would have gone. Ultimately in my mind, the heavy bombers are inevitably going to be more vulnerable, they have to fly slow, in big formations, in predictable paths, they can be detected far out, etc. Fast bombers and fighter bombers come much faster, can come more unpredictably, are smaller targets, and have much more agility. But the Germans did have good AAA. No doubt about that. And they certainly were adaptable.
In Japan, assuming you had aircraft carriers or somewhere to stage from, and good enough strike aircraft, I think the AAA wasn't as good so the strategy would have been easier to implement.