[MRM]

There are few people in this world who know more about residential windows than me. What brand are they? Jeld-Wen by any chance? Is there any fogging between the panes? Do you get much condensation? Do you get any drafts around the windows?

Based on what you've said so far I suspect you would see an improvement by making sure there's good insulation in the rough opening between the window frames and the framing more than buying new windows.

If you look at the corners of the glass do you see any kind of manufacturer's code or the insignia CIG? There should be a date, at least. Whatever the etching says, it might mean something to me.

The late 90s and early 2000s were a hard time for window technology. The materials and science hadn't quite caught up to the new energy codes and some manufacturers were pushing their designs and materials a bit farther than they would hold up over the long term.