It's true it would be impossible to ban all porn with any measure less than a great firewall, but it should be possible, and effective, to shut down professional porn studios and websites and corporations that purely exist to host and market porn, like MindGeek or Onlyfans. ISPs could be regulated or incentivised to take out foreign porn sites, it wouldn't be any more difficult than what was done to Kiwifarms. Pornography can't be eliminated, but it should be possible with measures like those to make it less mainstream. A porn black market doesn't really matter as long as it's hard to access and expensive, people willing to go that far were beyond saving already.