Good article. See my FR Home page for lots more on the real Russia-US collusion.
The basic reason that there is this pounding on the (imaginary) Trump-Russia collusion is not because of evidence, and not because the left is concerned about Russian meddling in US affairs. After all, the left has been courting Russia for decades. Rather, the narrative is born from a calculus that the right is concerned about Russian influence. The entire show is all about convincing the conservative base that Trump cannot be trusted.
It is a variation of the usual playbook. Take an issue that concerns conservatives, and make up some sort of “push-button” narrative to convince conservatives that their candidate has serious failings regarding that issue. Typically, enough conservatives fall for the narrative to swing the election towards the left. Once the damage has been done and the left has gained a little more power, the issue is dropped. The tactic reliably works; until all conservatives wise up to it and it becomes ineffective, the left will never stop trying to convince us that our candidates fall short in areas that the left could care less about.