If they can just consolidate control of the supply chain, they don't have to spend the eng-hours replacing their open source dependencies and/or forking them for internal maintenance.

It's the kind of thing that can be the net cheapest option in the long run (see also: Facebook creating and open-sourcing one of the most popular JavaScript interactive-UI frameworks, resulting in them having control of that ecosystem).

Seems like a much riskier move to me. Maybe this will just blow over and it will all work out for them, but I wouldn't feel certain of that outcome if I were them.

Agreed that if they had created and fully controlled the whole thing from the start, like your Facebook example, that would make more sense. But this aggressive addition of a new owner and removal of existing owners and maintainers seems very hostile and risky to me.