Let's not forget two specific things here: secondly, it was initially only about the current house; later, a conservatory is to be built behind this sofa/window; and firstly, the discussion at that time was further complicated by having to master a kind of magic triangle: namely a. rotating the dining table when more guests come, b. still having passage width for the terrace door with the dining table turned, c. simultaneously accommodating a music cabinet from Mrs. Berta Panislowski from Massachusetts. And doing so d. without having to acrobatically lean too much over the backrest of the sofa to water flowers on the windowsill behind the sofa, then led to a standard sill height for this window (if my memory still works perfectly, as I hope).
Well, the position of the conservatory was not defined; it could have been somewhere else (ideally where it wouldn't darken a window, so e.g. rather by the kitchen, etc.). But that would have led to a complete redesign due to the building envelope and setback limits. That certainly wouldn't have been so easy.
Yes, the thing with the piano still bothers me; it didn't have to be and I would have been much freer in the design. Or rather, it would have generally been better back then not to think so rigidly and simply allow more, at least to play through more variants (or also window sizes).
The standard sill height resulted from the fact that A) back then we found it annoying to see the back of the sofa from the garden and B) having a storage option for flowers etc. at the window. Giving up these two ideas at that time would certainly have been very difficult; or we would have had to think completely in terms of a modern house with floor-to-ceiling windows everywhere, which we did not do. As already said, unfortunately, windows were VERY unimportant to us back then, unfortunately. Today, they would be my most important point in planning.