That's the hitch. "properly sealed" single pane windows about never exist. Most were builder grade windows that had gaps around the sashes that I could fit a tri-fold pamphlet in-between. Very easy to demonstrate in older houses. If the window rattled at all, I usually had a pretty easy sell. Yes, the storm window typically provided all of the energy savings that the builder grade windows lost, but who wants ugly-ass aluminum storm windows on the outside of their house?
If energy savings is ALL you are after, then typically repairing what is leaking and properly sealing them will solve the problem. 99 times out of 100, however, the homeowner is looking for an aesthetic upgrade as well and replacement windows do that. Many times, homeowner is also looking for a maintenance-free exterior and a wrapped aluminum clad or vinyl exterior does that, too.