From my own work experience, the "dinosaurs" are the most valuable people in the whole org, because oftentimes understanding the code isn't nearly sufficient to make good long term choices. You very often need to know the historical context of why certain things were made in order to not shoot blindly in the dark with random changes.
Outside the world of foss there are for example multiple games that are highly respected that were all entirely or largely developed by a single developer. Rimworld, Songs of Syx, Dwarf Fortress, Ostranauts and many others. In a proprietary context I have myself deeply understood codebases and worked with others who have also.
That being said, you are arguing against a straw man that neither I nor the rust article propose. They are not saying maintainers need to understand all the code they are saying the person submitting a pr needs to deeply understand that pr. Why? Because they want to build a community who deeply understand various aspects of rust so when things need maintenance there are experts around who even if they don’t know a particular piece are invested in the success of the project and know enough to get up the curve and fix whatever needs to be fixed.
Fabrice Bellard is obviously a superhuman, but I expect even he relies on re-analyzing code he previously wrote and has subsequently forgot. This doesn't take him long because he knows his own conventions and proclivities. But I'm confident he doesn't "deeply understand" the entire code base. He has a shallow understanding of the entire code base. He has a deep understanding of crucial parts of it. And he has the ability to quickly re-establish a deep understanding of any piece.
Bellard hasn't worked on FFMpeg since 2003.
Now, if you have program that is highly complex, written by a single author who is not you, hinging on the ongoing goodwill and capability of that single author, that is a liability you do not want to have, operationally. Again, that is just how it is.