It's not an ethnic cleansing.

Ghettos by definition are mostly mono-racial. In America the word is usually only used for one race in particular. By all definitions it will end up just being an ethnic cleansing by the methodology you've chosen to use.

Oppressing criminals is a moral imperative to a just society.

No disagreement. This is a discussion on how and the obvious reality it will bring that is opposed to your own standards.

If a cop can't walk down the street without getting robbed, attacked or shot at

Welcome to what most of us in the South lived with our entire lives. The fact that they are in fact this stupid at times and will constantly provoke a justified armed response. Its such a well known problem we have had black media talking about it for decades. One classic one for you is Chappelle's "When Keepin It Real Goes Wrong." Or Boondock's "Nigga Moment."

The only thing that often keeps them from acting is cowardice, meaning multiple openly packing officers moving through. A single plain clothes guy is just going to get jumped from behind before he can call in or respond, and then everyone scatters once they find his badge. And you'll never find those guys because snitching gets you lynched.

If you want to put a few cops on every block to overwhelm the problem that's one thing, but that's not what your original proposition was.

then you keep putting cops on the street until it stops.

And the reason we don't is because when we even tried a fraction of doing so, the optics from the media kept getting so bad it was safer to just try to cordon it off and keep it in the box. Not to mention the safety of the officers themselves. Every few years going back decades now we have had a major news cycle all about a cop doing their job and a black guy getting killed out of his own idiocy, and it was enough to move mountains.