The only way to "fix" this problem is to completely abolish the institution. The great majority of cops, including all upper ranks, would have to be permanently removed. A new heavily emasculated entity could be created, stripped of all military overtones, with the guiding principle that the organization has no authority and no right to question its own limits or lobby for changes to them (except to impose more limits following even the slightest whiff of potential abuses, which must be reported immediately). Only people who want to serve under those terms, and will hire others of similar disposition, could be considered for membership in the new organization. Termination of anyone demonstrating less than total subservience would have to be immediate and unappealable.

Such an entity would not be recognizable as a "police force" to anyone currently living, as most are heavily militarized and populated almost exclusively by individuals with a pathological need to exercise absolute power over others. But yes, in principle such an entity could be created to replace what now exists.

There are two major problems: first, the individuals making up today's police forces won't just go away, any more than piratical sailors did following the end of the War of the Spanish Succession or the Ba'athists did following the fall of their government in Iraq. They're still going to be somewhere, seeking unlimited power by whatever means they can, lawful or not. Something has to be done to prevent them from reconstituting themselves somewhere else. De-Ba'athification turned out to be harder than it looks; abolishing an institution takes more than removing its members from the public payroll.

Second, as most scholars probably would accept, risk-aversion among politicians is an essential failure of representative democracy. It is usually impossible to find the political will to do anything that could lead to "headline outrage" later. This is a basic feature of this form of government; simply throwing out a particular group of bums won't address it. No rational politician would vote to reform the police in any radical manner because any subsequent "heinous crime" would be dumped on his or her doorstep (regardless of whether any police force conceivable could have prevented it), and of course the former cops would encourage this process and milk it for all it's worth. The rational politician is interested in maximizing opportunities for reelection, not the good of humanity.

The two analogous historical comparisons I might make, in which public institutions were replaced wholesale with the mandate that horrific errors of the past never be repeated, are the Bundesbank and the Japanese SDF. The Bundesbank has effectively been replaced after only a few decades by the pro-inflation ECB, and the SDF's pacifist restrictions, after decades of being whittled away, are now under discussion for outright removal. So the historical track record of "never again" isn't very compelling, nor is the historical track record of firing large numbers of armed and dangerous "public servants" with limited skills and a history of supporting one another through corruption and perjury. And we know that no representative democracy's politicians are ever going to support such an act. Your ideological commitment to the ballot box is laudable, but does not provide any real solution to the problem at hand, nor does it refute my essential characterization of today's police institutions.