Fritz Fend set the ball rolling, and once you have a proven concept it is MUCH easier for others to add improvements. I have regularly designed instruments which no other person or company has come close to even imagining, and have a reputation as a wildly eccentric maverick among drummers. Some of the big drum companies like Yamaha, Pearl and DW (Drum Workshop) have huge budgets to spend developing "new" products, which are invariably re-hashes of familiar stuff, but done to a very high standard and mass produced in hundreds of thousands. Fend was very keen on developing and improving the KR design, with more performance etc., and th budget he was working with on the TG 500 was far less than other car manufacturers spend designing their badges. With Messerschmitt pulling out of bubblecars and going back to aircraft (Messerschmitt Bolkow Blohm / MBB) Fend just had to knuckle down and do the best he could to bring out his last-ditch attempt to achieve publicity and sales.
I know only too well how hard it is trying to achieve good results when finance and resources are far less than satisfactory, and despite all the faults on the TG I still think it was a great achievement.
The engine was not fully developed, and the whole concept of Tandem seating was by then completely daft when it was comparable in length and width to Fiat 500s, Goggos, Minis etc, but with only room for 2 adults. The layout was fine to make a 3 wheeler stable and well balanced, but once you have 4 wheels that narrow body is totally stupid, and the whole concept is daft. I understand that Fend was desperate to develop a totally new small car that made use of it dimensions, but there was no money because of rapidly declining Bubble car sales; the brief boom ended suddenly, leaving no money for many makers to develop new products.
Perhaps that is part of the appeal of TGs, a shoddy path leading nowhere. But it is one of my favourite paths. and however flawed, like Morgan 3 wheelers, I still love them!