That was at first, then Putin started having them murdered. If you think about it, there isn't much of a reason for the individual that is allowed to have all the power to share it with anybody.

The more accurate model for the Russian government is a Mafia family with Putin being the Capo dei capi.

The murders are the visible symptoms of the various factions fighting each other.

Not Mafia but Vor... https://en.wikipedia.org/wiki/Thief_in_law

Mostly the ones that piss him off.

So my, admittedly distant, understanding of modern Russia is that the FSB and Oligarchs have formed a symbiotic relationship, with Putin as its fulcrum.

The FSB secure the oligarchs, and prevent them from being prosecuted for siphoning off billions from the Russian economy. These get distributed down through to FSB leadership as bribes. The whole thing stays loyal to whatever leadership coalition keeps it going. Putin has proved quite good at that.

This arrangement is also underscored by a sort of modern descendent of Chekism. There's an ideological component besides all the corrupt money making.

There were different cliques of oligarchs. Putin had the ones opposing him eliminated, and kept the ones that supported him.

this article is about NASA.. the history of corruption, authoritarianism and consolidation of power in private hands would be books longer than Balzac.