All three of these games have something in common, that being that they're all sandbox games. Any other genre usually has the game end with the final boss. While Terraria is more adventure focused than the others, the final boss should be the last thing in the game progression wise. If we keep adding more events and bosses that can only be fought or completed after Moon Lord, that defeats the purpose of having a final boss in the game.
I'm not saying it's the worst idea, that's just my opinion on post-ML content.