At worst (as in the most waste of time), the Russia investigation will clean up some corruption by a few minor-ish players like Manafort. It may catch up some more important figures if they sought to cover up for those under them. It may do more. Cleaning up corruption is good for the nation, and sending a message to those that try to cover up crimes by associates is also good for the nation.

Regardless, Trump is allowed to be upset. He isn't given a pass to libel a Senator because he is upset, though.