Choosing people interested in fact-finding could theoretically be zero-sum on a jury except that random people are not charged with crimes. In this same class, and it may be different now compared to 10 years ago, it was said that 90% of defendants plead out, and of the 10% of defendants who go to trial, 90% are convicted. Criminal justice resources are stretched thin, and it's in no one's interest to throw out a dragnet and haul in a bunch of iffy cases.
This lecturer also said most pleas and convictions resulted from pretty obvious evidence. It's not Atticus Finch or Perry Mason. It's not even Night Court. If a case isn't a slam dunk, the prosecution won't bring charges. So being charged is tantamount to guilt.
That's a nice story. In reality, it takes more resources to solve hard cases, so they won't get solved. Instead, some random poor bastard will get charged and will plea out.