Are you saying that Assange would just volunteer this information to the FBI? Even if they offered some sort of deal involving immunity from US prosecution, it's unlikely that Assange would have had any proof that the source of the emails was the Russian government. (Also, incriminating the Russian government in a crime can have negative effects on your life expectancy).

As for your statistics, it's worth noting that Mueller's investigation probably made a profit by uncovering millions of dollars worth of tax fraud[0], and it lead to indictments of or guilty pleas from 34 people and 3 companies.[1] That doesn't seem like it was a complete waste of time or money.

[0] https://www.inquisitr.com/5213408/robert-mueller-investigati...

[1] https://www.vox.com/policy-and-politics/2018/2/20/17031772/m...