RadonGOG: But I can´t understand why we should stay at 640x480 while we are able to perform even 1920x1080 on LowCost-Chips!
That´s just a waste of your monitor! (640x480 on a modern monitor looks blurred)
Because that's how pixel art works. If the game was running at 1920x1080 then everything would be really small and you'd be able to see further. If you made everything higher resolution instead of small then it wouldn't be pixel art any more. Pixel art and 2D are not exactly the same thing. This guy is a pixel artist, not a traditional 2D artist/animator (what you'd need for a higher resolution).
Also I think you're missing the difference between internal resolution and external resolution. The game's internal resolution is low (256x192 if I remember correctly), but the game is almost certainly designed to have an 'external resolution' of some much higher multiple of that, so as far as your computer and monitor are concerned, the game is probably running at 1024x768 or 1280x960 (4x and 5x respectively).
RadonGOG: And of course gameplay is king.
But even Turrican had more fluent annimations than this one AND Turrican had been graphically progressive at it´s time:
At least I think they should implement a feature to avoid blurryness caused by the upscaling:
Such as only allowing pixel addition in aspect ratio and than filling in the black borders with some athmospheric elements!
I'm not sure why you think that there isn't such a feature already. At the very least it appears to have integer multiple nearest neighbour scaling, the video doesn't look particularly blurry.