80% isn't everything. And I doubt a single technique (other than a straight punch) really fills that much. That sweep won't cover a tackle, shoot, kick from range, or someone whose footwork puts them in range before you can set the sweep. I like a foot sweep, but it's not that universal. If it were, we'd see competitions where the first move was a successful foot sweep over and over.When your opponent punches you, he has to put weight on his leading leg. You can either kick that knee, or sweep that leg. The opportunity will always be there.
If you are good at foot sweep, you can handle almost 80% of your problem. Some techniques are more useful than the other.