You are correct, football keepers make a much lower amount of saves - but goals tend to be fewer and hence "heavier" in football, so having a bad keeper will still likely result in a loss. And it's still the sort of role no regular player wants to do - which results in some funny differences at the cultural level.

At the very least, I guess this concept could work as a feature for all those apps people use to arrange 5-a-sides.