1. If the police catch every child sex trafficker selling on Backpage, then there won't be anymore posting to Backpage. The ones left will either get more savvy, or already more savvy and weren't using Backpage.

2. If they don't catch a large enough percentage of the traffickers, then the ease and reach/audience that Backpage provides will still entice people to use it.

Sure, you can use it while it exists (to find and catch traffickers), but it either won't exist for long, or continuing to operate will still allow some amount of trafficking to successfully happen.