The lack of interest shown by the window supplier is very poor but perhaps typical of the slippage in commercial standards these days. I think it unlikely that the timbers were defective when the windows were manufactured ten years ago. A much more likely cause is that a gap has formed between the glass and the glass support (beads or putty) and that rainwater has trickled down this gap over the years, causing the rotting of the bottom member of the window. If that is the case I don’t think that you will get any satisfactory action from the supplier.
But, it does sound as if the windows have been well maintained, so if the damage on the three windows is isolated to an exposed area of the property and there is a clear design, material or workmanship fault it might be worth checking if the property still benefits from a 10-year structural warranty (if it ever did at all). Obviously time frames will be tight, but at that time most warranty providers covered defective weatherproofing, so you may have some success there.