Anyone in software engineering will tell you that as you get more senior you spend less time writing lines of code and more time planning, designing, testing, reviewing, and deleting code.

This will continue to be true, it's just that there will be less juniors below who's whole job is to produce code that meets a predefined spec or passes an existing test, and instead a smaller number of juniors will use AI tools to increase their productivity, while still requiring the same amount of direction and oversight. The small amounts of code the seniors write will also get smaller and faster to write, as they also use AI tools to generate boilerplate while filling in the important details.