Yes, that is a problem. No single solution is perfect, and VPN blocks are only one tool in the toolbox. Every instance is different, and admins pick the tools that they think will work best for their situation. VPN blocking happens to be a tool that seems to have worked well for lemmy.world.

Instances that lack moderation and have a history of being used by trolls tend to get the nuclear option: they are defederated by the big Lemmy instances. That’s not ideal at all, but the tool set for Lemmy moderation is very limited right now.