I think there's a point you may have overlooked: The outside of any container holding radioactive materials becomes more difficult and dangerous as time passes.

I think there's a point you may have overlooked: The outside of any container holding radioactive materials becomes more difficult and dangerous as time passes. Even a sealed container. Storing containers accessibly but yet securely and designing them such that they remain safely sealed for 100 or 200 or 300 years, yet can be handled then, is a very big problem. AIUI a more difficult problem than finding stable rock that's been undisturbed for a long time.