Here's the reason Donald Trump is toast, and will resign the presidency: When he proved he cannot keep state secrets he became a danger to the security of the U.S. It was the final straw. Those around him could no longer defend that he is benign and others will run the government putting him up as a front man. He is a child who cannot keep his mouth shut.
So Rod Rosenstein decided, along with others I presume, that he must go. They may or may not prove collusion with Russia. As I said in a January post, Trump can be impeached at anytime, due to his transgression against the emoluments clause of the Constitution. It's just a matter of getting a general consensus that he must go. That consensus is now occurring, particularly in the security establishment.
Robert Mueller III can now investigate anything he cares to investigate. That's the reason it must be a special prosecutor, and one whom everyone believes acts with integrity. Everyone now knows that Trump cannot be President, and Mueller has the assignment of getting rid of him. Whether it's the Russian connection, or the emoluments clause, or some other crime Donald Trump has committed, he will eventually be told "You have a choice. You can resign and we'll make up a reason, like 'the good of the country.' Or we will indict you and you'll face a trial, shaming, and go to jail. You choose."
He will resign. He's too dangerous to allow to stay.