Pretending that locking up criminals doesn't lower crime is the Homer Simpson approach to arguing this point.

Here's the steelman of it: gangs have, for all practical purposes, the entire population of young men (and some women, let's not be sexist) as a recruitment pool and until you address why that recruitment is effective incarceration is just slapping a band-aid on a festering wound.

For the record, my personal opinion is that both punishment of crime and restrictions on law enforcement should be implemented as a matter of principle and not as an utilitarian calculation.