The "original windows" in this context are windows 700 years newer than Notre Dame that were themselves part of a significant modernisation, and only a small pa

The "original windows" in this context are windows 700 years newer than Notre Dame that were themselves part of a significant modernisation, and only a small part of the windows of Notre Dame (about 5% of the windows are being replaced). The replacement windows are also made by a studio that has a history going back way past the windows being replaced, and a history specifically of restoring historic glass - I very much doubt they'll need replacement within 20-30 years. The gradual renewal over centuries of renovations and restorations is itself part of the historic legacy of Notre Dame.