We witness here in the Netherlands the cultural slaughterhouse since these last 30 years and the most grotesque aspect is the gouging out of the eyes of buildings replacing them with artificial ones. Not different from Duitsland but they never had such nice windows there to begin with and the same can be said for France. There is no compromise room in this question, which makes it really a simple matter: Old windows have to be saved to the fullest extent.
The distinction of a Dutch window from a certain period is they had been conceived of as bringing the inside out for public inspection or at least providing for the evidence that the inhabitants surely had no secrets, nothing that could not bear scrutiny in other words a port into the souls of the house’s occupants, their righteousness and purity, the windows open to the street and stretching practically to the eaves they pressed the passer-by to look in and see for themselves the orderliness of the household and by extension the people living there who had had great confidence in there complete exhibitionism. Even to this day a stray glance at a passer-by will likely lead to a defiant yet ever so defensive invitation for open inquiry into the observed persons piety. They have nothing to hide and dare you to question this fact.
More positively, leaving the Calvinistic oppressiveness eating our dust, the windows have developed remarkable proportion and give the gables a sense of openness and lightness, just as windows should. All the more cause to reject the disproportion and heaviness of thermopane replacements.
I manage to somehow capture some of the character of the old glass as I took this shot looking out which you can see as the angle steepens when you look at the upper left corner of the window.
Because the window frame had rotted, and would twist when lifting it open I once broke the pane so after I fixed the wood I took the glass to have it fused back with a strip of lead in the way normal leaded glass gets done and flipped it bottom to top before resetting.
There probably is a trick for making photos of glass because mine sure don’t give a very adequate image of the old pane on the left with its wavy surface that distorts any image on the opposite side and its green tint that comes from adding lead to the pre-molten sand mix, and the trapped air bubbles. I did manage to catch one prominent bubble though, sorry for you that you can’t get the in-person effect.