Nice job in a few words. Thanks to Mollie Hemingway.
Mueller presumed that Trump, given his nasty remarks and the number and range of people who hated him, must have done something seriously wrong. Furthermore, his election is difficult to explain unless some bad things were done by and for him. So Mueller presumed guilt, and went looking for people who had done something wrong, and could then be threatened with extreme, almost unheard of penalties unless they tesfified against Trump. Mueller knowingly or unknowingly was led down a garden path by Trump haters in government, and by the Steele dossier which itself was cooked up by American Trump haters, including Brennan, and a handful of dubious European "allies." Hillary paid for information from foreigners, some of which (for all we know) could have been fed to her by Putin wishing to disrupt the U.S., sow division, etc.
Trump collusion with Russia: nothing so far.
Trump colluding with somebody, anybody, in order to win in 2016 in some improper way: nothing so far.
Trump hiring/associating with dubious people, some of whom are guilty of crimes unrelated to the 2016 election (and mostly unrelated to Russia): yes, but not a matter for a big-time prosecutor. Let the voters decide.
Trump's business dealings, financial paperwork, or something from before 2016 may turn out to be bad somehow? No president has ever been investigated or impeached for such matters--from before his time of public service. This continues the Mueller thread: if someone does something bad, which is normally punished lightly if at all, and then at some point performs some public service which is somehow on the wrong side, e.g. hostile to poor sick old Hillary, then it is right to bring down entire buildings on this person in order to find out some truth that might not otherwise have been known. Judge Sirica, the Senate Watergate committee and Woodward and Bernstein, all at once. But it is the public service that triggers all this destructive response, allegedly based on "investigation of the law." If it had not been for the public service, the investigations and prosecutions would never have occurred. Probably the oldest criticism of all the special prosecutors is that they make ordinary people, some of them let us say a bit below average, pay a terrible price for public service.
VDH says let's have another special prosecutor, focussing on known (not imaginary) misdeeds by officials at the Department of Justice and FBI. The new person and Mueller might investigate each other. All documents must be released, unredacted. The point is not to prosecute people, leave them homeless and all that fun stuff, but to find out the truth for a democratic public. Of all the people who were no doubt trying to influence the outcome of the 2016 election, who did what? Who was the worst? Compared to this, cheating on taxes, other kinds of fraud, lobbying in Washington without a licence (hah hah), violating the Logan Act (hah hah) are all fun, but may be of no actual relevance to public policy, who is doing a good job, and things like that.