The impact of the strategic air war is greatly debated but the growing belief is that it did not return the value for the resources committed:
1) German production continued to grow. Several studies during the war and after the war determined that the bombing campaign destroyed the building - primarily the walls and roof - with little impact to the tools. This allowed the Germans (and Japanese) to either get back on line quickly or move the tools and get back on line quickly.
2) While limited disruptions in supplies and equipment happened, in general, the Germans never really suffered due to a lack of production due to industrial capacity.
3) The attack on marshaling yards, rail ways, and rail centers outside of the tactical zones had little, if any, long term impact. Despite all the hype about this being a critical component showing the effectiveness of the air strikes, this doesn't show up in the post war reports. In the tactical war zones, the FB and low level bombers really made a mess of the German day light movement.
4) This changed after the firebombing of Dresden. In that raid (and the firebombing in Japan), large areas were literally destroyed. Dropping iron bombs didn't work, dropping firebombs did.
5) The strategic air campaign contributed to a shipping shortage after Normandy. The shipping space ended up being really tight after the invasion as the demands increased dramatically. And the Strategic Air Forces took up a lot of space in hauling bombs, spare parts, and ammo to support the effort. At times, it required a trade off between shells for the big guns on the ground or bombs for the big planes.
6) The Strategic Bombing campaign destroyed the German Air Force as an effective fighting force. It took away any space for it to train its pilots, required it to use it fuel up in defense, and literally ended up shooting it out of the air. Nearly 1/3 of all combat losses suffered by German Air Force in WWII was caused by Allied Strategic Bombers (not escorts - bombers).
7) Germany's devoted nearly 450,000 people to man the AAA guns. While nearly 90% of these would never have served in the front lines (too old, women, or really young boys), the number of guns, amount of shells, and training were missed, especially on the Eastern front.
8) And, to be fair, it did delay production at time and did delay transportation at times. Whether these delays represent critical issues is open to debate.
9) And it added to the British morale in knowing that Germany's were under attack in Germany.
Finally, a number of people have suggested that these resources be devoted elsewhere. The question would be where? The Allies had all the tactical air support that they needed. Most of the fighter support would still be required as the German Air Force would NOT have been destroyed over Germany defending against the bombing campaign, and moving these men and equipment to ground forces doesn't really look like it would have ended the war any faster as logistics was the primary brake on the Allies after the invasion.
At the end of the day, I'm not sure you had a better "investment" in resources. Could it have been deployed more effectively? Sure, with 20/20 hindsight and given the information we have now. But it was the first "truly strategic bombing campaign" in history.
In the end, it destroyed the German Air Force - which was a big help to the Allies. It provide a major air front that suck up resources that the Germans could have used elsewhere more effectively. And it did impact production although not to the extend that the Air Forces at first claimed or set out in their objectives.
As far as ethics goes, not a question I considered here. WW2 and WW1 both represented total war - and that includes attacking the means of production for war material.