> There is no place in the world geologically stable for the required period (we have no way to know)

This can be said of anything, which is IMO a defeatist attitude. Nothing we do is guaranteed to not cause harm. Likewise, our inaction could also cause harm.

For the specific question of stable geology, past performance can help predict future performance. We can make educated guesses. If it has been stable for the last million years, it's a good be it will be stable for the next 10,000.

> We have no way to communicate with people about the dangers for over 100_000 years.

The need to communicate the dangers over vast timeframes presupposes that society has fallen into ruin, and our record keeping has been destroyed. I will argue that if that has happened, it means countless billions will have perished, and people+or lizardmen) living primitive lifestyles will have far bigger worries than and increased cancer risk.

> The political impetus is to get future generations to pay for our current consumption

We are paying for our current consumption of fossil fuels right now. How many deaths from climate change over the next 200 years are appropriate to ensure the safety of an unknown number of people thousands of years from now?

https://en.m.wikipedia.org/wiki/Vitrification