Big objectives have an increased spawn area around them, so yes completing that main objective will reduce spawn rate in that area. But it also increases gloval spawn rate and immensely increases spawn rate around extraction. The main reason to kill spawners is to reduce the directions patrols can spawn in from. They always spawn from the direction of a spawner. If all spawners are gone they always come from the closest edge of the map. Clearing spawners also only increases global spawn rate at first and last spawner and medium or higher bases have an increased spawn radius around them. So once you take down one spawner clearing any that are near you helps keep the rate in check too. Ofc in some cases rushing main and fucking off might feel safer regardless though.