> You need a very different mindset to write in JS (or TS), in Rust, in Rocq, in Esterel or on a Quantum Computer.

"Senior", "principle", etc. are not about your ability to write. They speak to one's capacity to make decisions. A "junior" has absolutely no clue when to use JS, Rust, or Rocq, or if code should be written at all. But someone who has written (well-written) tests in JS, and maybe written some types in Typescript, now has some concept of verification and can start to recognize some of the tradeoffs in the different approaches. With that past experience in hand, they can begin to consider if the new project in front of them needs Rocq, Dafny, or if Javascript will do. Couple that with other types of experiences to draw from and you can move beyond being considered a "junior".

> You might be able to have "seen it all" in a tiny corner of tech

Of course there being a corner of some sort is a given. We already talked about management being a different corner, for example. Having absolutely no experience designing a PCB is not going to keep you a "junior" at a place developing CRUD web apps. Obviously nobody is talking about "seeing it all" as being about everything in the entire universe. There aren't that many different patterns, really, though. As the terms are used, you absolutely can "see it all", and when you don't have to wait around for the season to return next year, you can "see it all" quite quickly.

In my experience, levels are more a question of delivery and trust than of technical skill. If a Senior is off down a rabbit hole, that's them doing their job. It's almost a defining feature of the level down (SE2, or local equivalent) that if they're off down a rabbit hole then something's probably gone wrong somewhere: if you could trust their judgement, they'd be a senior already.

(Yes, that's circular and overly reductive, don't take it too literally)

Where I've worked, "senior" has also meant being a final authority: more junior folk will come to you with questions about your team's codebases, and more senior folk will too. You might not have anyone else to ask that kind of question of.