Homeowners feel a draft near a window and conclude the window is the problem. Frequently the leak is not through the window at all, but around it — at the joint between the frame and the rough opening, behind interior casing that was never air-sealed.
Within the window itself, the dominant leakage paths are predictable. On double hungs it is the meeting rail, where two sash interlock and where decades of paint buildup and hardware wear have opened the interlock. On casements and awnings it is the compression weatherstrip, which loses resilience over time, and the operator adjustment that determines how hard the sash is pulled into the seal.
Glass, by contrast, leaks essentially nothing. It is a continuous impermeable surface. Replacing glass to solve a draft addresses the wrong component.
This is why restoration frequently outperforms expectations on air-tightness: rebuilding weatherstripping, correcting hardware adjustment, and air-sealing the frame-to-wall joint attacks the actual leakage paths rather than the visible one.