Because we have so many different software products, services, languages, and steps to production for any given piece of software, there is no such thing as a senior developer anymore (…) Or rather, there is no such thing as a senior developer who has both the depth required in earlier development environments, and now, the breadth required from the modern software stack.

What distinguishes them now is breadth and, I think, the ability to discern patterns and carry them across multiple parts of a stack, multiple stacks, and multiple jobs working in multiple industries.