I really think that's an oversimplification.
There are always the pro and con arguments about software mono culture and there are good points on both sides, and in this case there is indeed a gigantic cost to maintaining more than one implementation -- no one is debating that. Also, an across-the-board evaluation would probably find Firefox technically inferior at the moment, although I think that's more of an interesting debate and also a measurement where I feel the gap has lessened these last few years. My disclaimer would be that I'm a long-time user of both browsers, but ethically on Mozilla's side.
However, I think the following
> and to an implementation which satisfy every interests (the ones of Googles and the one of Microsoft/mozilla)
doesn't put nearly enough emphasis on exactly how different those interests are. Google's main source of income depends on user surveillance and privacy intrusions (to use some loaded wording), and while Mozilla's indirectly does too through their sources of funding the two entities have very different goals and core beliefs.
That in itself doesn't preclude sharing an implementation, but thinking that those differences won't (and don't) cause conflicts of interest that affect both the evolving of the standards and how the actual implementations work would be, I believe, a big mistake.
Just saying "ensure the respect of privacy in the chromium source code" and that they should maintain a set of patches or a "micro fork" I think ignores the level of control they will be (or rather won't be) able to exert over the Chromium code base. I don't think Opera, Brave, Microsoft, Mozilla or anyone else that decides to either be a downstream user of or active participant in Chromium will be able to keep Google from doing whatever is in their best interest with the code base. Additionally, I think keeping patch sets (that won't end up being very complicated and will slow down or put to a grinding halt independent development) won't be able to make up for the implementation differences caused by the differences in goals. Keeping patch sets can be extremely painful.
> Chromium is open source and has those two main actors: opera and Microsoft.
I really apologize for my tone, but... Please. In what regard are they main actors? Certainly not to the extent that they have any significant control over the main direction of the Chromium code base or any power to stop Google from completely controlling it.
The core beliefs and goals of the entity exerting the main control over the code base matters a lot more than your post makes it sound like. And if Chromium really were the only code base able to browse the modern web, then that would also put the standardization of the World Wide Web more or less in Googles hands.
Also, for someone asking for better "intellectual riguour", you do a pretty so-so job at showing your understanding of the arguments of the people you don't agree with.