What I don't understand is that for most SE jobs on mature codebases, writing code is maybe 5-10% of the work. And it's that 5-10% coding that keeps you 'grounded' in the code and keeps your tacit understanding up to date.

Get rid of that and you get a basically invisible productivity increase (call it 10% to be generous) but you've now lost tacit understanding. No amount of half-assed code review (or getting the LLM to generate "code quizzes" etc for you) is ever going to substitute for the tacit understanding you get from doing the 10% of coding yourself.

I really just don't get it. I mean, I can believe that using coding agents gives people a rush of power and fleeting illusions of increased productivity, but it's a mirage.

A lot of the comments here are trying to find some middle ground, i.e. trying to find some way to preserve tacit knowledge. But it seems to me that it's a waste of time. You either need to write the code yourself or completely give up on even reading or attempting to understand it. I just don't think there's any middle ground. Any well-intentioned attempt to preserve understanding of the code will fall afoul of human laziness and understanding drift