Remember Clinton. Impeachment made him way more popular as many people felt there as an overreach. I wasn’t one of them but his popularity went way up after the House impeached him.
If the Dems impeach Trump, I think you would see the same thing.
If the Democrats win back the House and impeach Trump, the media will be cheering them on. How the public will react, I don't know.
Mueller saying that Trump is not a criminal target is nothing to be relied on...he can always change his mind. Maybe the idea is to induce Trump to let down his guard.