I've never worked anywhere where the role of a Sr was to glue together a bunch of small pieces written by a team of Jr devs.

I've only worked places where Jr's were given roughly the same scope of work as a mid-level dev but on non-critical projects where they could take as much time as necessary and where mistakes would have a very small blast radius.

That type of Jr work has not been made redundant - although I suppose now its possible for a PM or designer to do that work instead (but if your PMs are providing more value by vibe coding non-critical features than by doing their PM work maybe you don't really need a PM?)