that while there's weight to the separation of powers argument, there's a YUGE practical concern with the conclusion that presidents are prosecution-proof:

Suppose the president actually kills someone, and the evidence is incontrovertible. Suppose, for example, Trump shoots much-loathed reporter Jim Acosta on live television during a press conference. The OLC opinion says he can't be investigated, arrested or prosecuted for it; he just gets to carry on as president as if nothing had happened. How would the country be able to tolerate that? How can it be said that we just have to wait until the House impeaches him and the Senate convicts him? Of course, the process would probably be expedited - maybe it would take weeks instead of months - but in the meantime, what damage could the president do, knowing that he'll eventually be turfed out of the WH and prosecuted? Could he hide the body in the Rose Garden and dispose of the weapon and claim that it was an imposter that did it? Take off for a country with no extradition treaty (Russia?) before he can be impeached or prosecuted? Kill more people? Set fire to the White House?

Of course, this is an extreme example of a hypothetical that's not likely to happen (Trump would never do his own wet work), but it does make the point that a blanket holding that presidents are immune from prosecution could lead to a result the framers of the Constitution never would have intended.