If they have money they have power

Mikhail Khodorkovsky was the richest oligarch in Russia. He criticised Putin in 2003 and spent the next 10 years in prison.

The other oligarchs learned the lesson.

https://en.wikipedia.org/wiki/Mikhail_Khodorkovsky

If Putin is against you, no money on the planet helps. He will have you killed in England.

It would require many oligarchs to form a conspiracy and remove him and several others from power. I have no insight, but my feeling is not enough people trust each other to do it. One leak or the slightest mistake and they are all dead.

This only works if they aren't going to die in nuclear fires. Once this gets to the point enough think that crazy Ivan is going to really kick off a global thermonuclear war his own protection fails.

No one has tried retaliating yet.

Set up a $100M 'assassinate Putin' fund and watch the fun.

I have no deeper understanding about Putin's inner cycle. According a detailed news article yesterday there are only 2 oligarchs left there. He prefers people with way less money these days because they are easier to control.

Most oligarchs are completely silent on politics, some are even in prison for corruption. Which of course is not difficult to accuse anybody rich or powerful in Russia. They probably don't have to dream up the evidence like in other cases of political (in)justice.