He knows perfectly well there is zero chance JS is going to be retired. It certainly has some serious flaws but the ecosystem is only growing richer by the day. Typescript plus React/Svelte/Vue etc is an extremely productive development experience.
In order to displace an entrenched technology it's not enough to be marginally better. You need to be an order of magnitude more productive to get people to pay the switching costs. None of the proposed JS replacements meet that criterion.
You basically just summed up what he said. He's referencing the security model and defining a common interface across browsers. Sounds order of magnitude-ish.
it will probably be replaced as part of a paradigm shift on the order of the web itself. what that will look like is hard to say, but maybe it will be a move towards more privacy oriented computing or high level specification languages for self-programming computers.
In order to displace an entrenched technology it's not enough to be marginally better. You need to be an order of magnitude more productive to get people to pay the switching costs. None of the proposed JS replacements meet that criterion.