It would be a disaster of epic proportions is Mueller and his coterie of kangaroos issue an indictment against anyone. Especially if it is Don Jr. or Jared or even against Trump, if he “happens” to find some financial issue he wants to twist. Issuing a pardon at that point is not a good option, and you can bet that every bit of info will be leaked to the press, to keep headlines coming in, day after day, week after week, drip after drip. The whole thing is a an effort to keep Trump in check, not to really investigate any wrongdoing, and the sooner it is stopped, the better.
If an indictment is issued, then at least you know the nature of the alleged "crimes." If Mueller is fired and the whole investigation is shut down, then the rest of Trump's life will be marred by rumors and outlandish media reports with allegations of crimes that never happened.
... if he "happens" to find some financial issue he wants to twist.
That would be the grounds to terminate him. His directive under his appointment as special counsel limits him to investigating matters related to the 2016 election. Even if any of the rumors about alleged Russian financial connections to some of Trump's real estate projects years ago are true, the lawyers for Trump's family members have correctly pointed out that the statutes of limitations for any alleged wrongdoing in those matters have long since expired.