The time frame is too long. It's too long for natural causes and far too long for human civilisation. The earlier we stop producing this crap, the better.
Sure, but that's why I suggested deep borehole storage - there is no known geological process that can bring up material stored at 5km+ depth into the surface on any sort of human timescale. A mine, even at 700m depth, is just too shallow for long term storage.
....and? If you wanted highly dangerous materials to use for some evil purposes it's 1000x easier to just manufacture some poison rather than dig 5km deep. I'd argue that making radioactive isotopes in an accelerator is still easier than digging. Not sure what your concern is here.
You just don't know.
The time frame is too long. It's too long for natural causes and far too long for human civilisation. The earlier we stop producing this crap, the better.