Something that almost the entire population uses all day is only using 5%? Sounds incredibly efficient. Repacking fonts to be smaller sounds about as useful as plugging a hand crank generator in to the power point.
For web developers that 5% is our responsibility, and we have the means to change it in ways many people cannot.
As for fonts — yes I'm sure it would make a big impact. Font files are a decent chunk of average page weight.