> If Google or Microsoft ever decides to just drop Chromium for whatever reason the code this time around is open, so someone else can just pick it up, and users can migrate to that.

Not really. Let's imagine for one second that Google switches to Mozilla's Servo or a secret in-house next-gen engine. Who could afford to suddenly hire the 200+ engineers it takes to maintain Chromium?

But that's not the real problem. The real problem is that Google will have de facto control on all web technologies. Chromium is open-source but has a closed governance, so nobody can say "no" to a Google patch, while Google can say "no" to any outside patch. This means that Google can, if they so decide, leverage this monopoly against the competition.

In theory, Google would have the technological means to kill the web presence of Facebook, Netflix or pretty much anyone with a single commit, which would automatically be propagated to Edge, Opera, Brave, Dolphin, ...

Similarly, Google would have the technological means to decide which credit card you could use to pay on the web (at least through WebPay).

I don't think that Google would do such a thing, but that's a lot of power in the hands of a single entity, exactly the problem that we witnessed at the time of IE6. Also, Google has, at times, be known to muscle in other browser's territory by using Google Search/Google Docs/Google Hangouts/..., so the temptation will certainly be there.