20% is a somewhat arbitrary figure. The Red Sox won the 2004 World Series with 16% of their salary taken up by Manny Ramirez which didn't seem to hurt their cause. I guess their are other similar cases, but I don't have time to look them up.
I get the point you are trying to make, but this team needs something to build with and a young player who plays a position that is relatively safe isn't a bad idea to me.