S10 E12 (The Doctor Falls) is the end of (Modern) Doctor Who. Such a perfect episode epitomising the character, and closing an arc for one of the longest villains. He even 'dies' at the end.
Everything since then has been badly written and purposefully disrespectful to the cannon and the audience, and has wasted so many fantastic actors.
This is 100% the answer, the only solution to the model decay from LLM outputs overwhelming the web is to start collecting data IRL.
This is also why companies like OpenAI are desperately investing in 'AI wearables' that no-one wants. They have to get the unpolluted data from somewhere, and recording real conversations will at least mostly have come from actual humans instead of AI.