I would whole heartedly disagree with you. Perform an air intrusion test or just use a thermal camera and you'll see why windows from the 1940s are not good compared to modern windows. It's not just the glass itself, it's also how the window frames were installed (i.e. without insulation around them). Storm panes don't do anything to help that. And the idea that anyone would need to replace modern windows "every 15 years" is baffling. Modern windows are going to last a lot longer than that, and more importantly, they'll function a lot better. These modern storm panes are permanently installed, meaning you can no longer open the window. That's a deal breaker for most people who like some airflow after a long stuffy winter.