A cassette being lighter than a freewheel would be relevant if the freehub body weighed nothing. But since it, uh, weighs something, that's apples to oranges. A more meaningful weight comparison would be a freewheel+hub with a hub+freehub+cassette. The difference would be marginal. Performance and maintenance are another story, cassette technology wins hands down.

Compare a 40-year-old steel frameset with 5-6 digits of km with a 40-year-old carbon frameset used the same distance (30-40 years from now). Racers obviously don't need to care about longevity.