Mueller’s probe began as and at bottom remains a counterintelligence investigation designed to learn about and thwart Russian interference in U.S. elections. That task would continue even if there were no defendants to prosecute. Trump would be especially hard-pressed to shut it down, since so many people in his Administration have acknowledged the reality of the Russia threat.
More significant, any damning information Mueller has amassed is sure to come out–in a report, or by leaks or congressional subpoena.
This is the vital consideration. Mueller was never going to indict Trump. He is not allowed to, and if he tried, he would face a steep and lengthy uphill battle in court. His main goal was always to find out what happened and disclose it publicly so that Congress, by follow-up investigation and possibly impeachment, and the American people, through pressure on Congress and elections, can, if the evidence warrants, decide Trump’s fate.