Would it be feasible for Google to close the source of Chromium

They can switch to a new licence, but it would only apply to future contributions, not what already exists. So if they did that, the result would be multiple forks. MS would need to maintain their own fork for Edge.

The ultimate question would be whether anyone had the soft- or hard-power to stop them if they wanted to, which might be doubtful. But like @Monologue said, it wouldnt be in their interest. Keeping everyone tied to them and their services is far more profitable than actually using hard power to stomp out projects that steal comparatively little from their profit margins. But profit margins are profit margins, so further changes to make Chromium/Chrome even more of a spyware while booting out forks is just a double win.

why would they want that? it is better for them this way since they ensure that any competitors will use their engine (which they decide what goes into the code) simply forking it wouldn't be that possible either because you would need to be able to update it regularly to fit the current web standards effectively competing the budget and resources of google

firefox is literally the last bastion of free internet, use the furry be happy