One thing we need to recognize is that the aero engineers in the employ of the major German aircraft companies were for the most part experienced members of teams that had already produced successful combat designs that proved themselves in actuality.
That said, it doesn't mean there weren't some fanciful designs that made their way to paper. This was not unique to Germany. Every country's designers had all sorts of "radical" layouts on paper and in prototype stage at times. Every see what the original Lockheed concept for their jet was? Pure 2nd generation Luftwaffe jet!
We need to realize that in those days when a concept was put on paper projected performance was approximated by using all the normal factors in play- drag, thrust, weight and so on. It was all slide rule. No computers. Sure estimates coulda, woulda been off but that is what development and testing are for. Designs morphed at times as initial flight data was recorded. Modifying a design is/was no sign of weakness.
Often when an Air Ministry requirement came down specifying, say 4- Mk 108s, that doesn't mean preliminary designs put on paper would have always accomodated them. Perhaps an initial layout would have only had space for 2. Perhaps the RLM would have compromised. Most certainly there was little doodling on the company's time. For the most part if we see a layout with the slimmest amount of estimated performance data that concept was something that was a direct result of an RLM spcification workup or the company's directive to conceive a viable craft.
All country's governments and military institutions laid down conceptual specifications for aircraft and other weapons they conceived. We must remember those details are arrived at by comittee. A lot of "wishes" are ultimately encompassed in them. While they say "we want a bomber that can travel 300 MPH with a range of 6,000 miles. Yeah depending on the time in history relative to the known technological data base it was easier said than done. So XYZ Aircraft company laid their best design and it was accepted yet the prototype did 280MPH and only hit 4,900 miles range. Was it a failure? Were the original performance estimates bogus? No. The estimates were made with the best information available at the time. When adjacent tech actually exceeded expectations a resultant aircraft would exceed the expected performance. That might be due to something as simple as a breakthrough in engine tech.
While we sit 65+ years hence and carp at the validity of what was then state of the art engineering it is easy to lose perspective. We must remember these guys had already been part of teams that produced planes like the Bf 109, FW 190, Ju 88, Me 262 and so on. Simply because someone with a smattering of aero engineering today discounts a WW 2 design as frivilous we need to assign weight ot that comment. How many successful modern combat aircraft has this person worked on? Yeah, none.
The advanced German aero designs varied in their feasibilities relative to their estimated performance. Simply because a certain design was not tangibly possible in 1945 doesn't mean it wasn't validated as production technology, metalurgy and powerplants caught up years later. The Russian aircraft industry in post war times was built on the backs of Germans from designers to production workers that were spirited away to the USSR for a decade or more forced to produce what the Russkies demanded.
Humongous amounts of prototypes, blueprints, production tooling, whole factories and people were taken to Russia. While the Soviet aero industry had their own stars we need to realize that the wealth of caputured German information and hardware made the rounds of all the design bureaus in the USSR. They got the Lion's share compared to the other Allies. We acknowledge that the F-84 and F-86 designs were modified at the urging of "our" Germans to employ swept wings for now obvious reasons. Yet many of us refuse to imagine stand out designs like the MiG 15 were not influenced in some degree by "their" Germans.
We must remember an artist concept is no more than a wishful fantasy. It's artwork. A designer concept is the result of envisioning a layout envelope based on input from a source like the Reichs Air Ministry and creating an initial workable idea.
Every plane ever came from a conceptual drawing based on desired requirements for performance. Drawings with projected performance based on the basics of power, weight, drag and such become blueprints that evolve through to until metal is cut. Even then modifications took place until the best plane could be produced given all factors involved.
Germany had no monopoly on radical designs considering the bizarre stuff to come out of other countries over the years either.