Personally, I wrote 200K lines of my B2B SaaS before agentic coding came around. With Sonnet 4 in Agent mode, I'd say I now write maybe 20% of the ongoing code from day to day, perhaps less. Interactive Sonnet in VS Code and GitHub Copilot Agents (autonomous agents running on GitHub's servers) do the other 80%. The more I document in Markdown, the higher that percentage becomes. I then carefully review and test.
If you look, there are people out there approaching this stuff with more objectivity than most (mitsuhiko and simonw come to mind, have a look through their blogs, it's a goldmine of information about LLM-based systems).
I'm in a tiny team of 3 writing b2b software in the energy space and claude code is a godsend for the fiddly-but-brain-dead parts of the job (config stuff, managing cloud infra, one-and-done scripts, little single page dashboards, etc).
We've had much less success with the more complex things like maintaining various linear programming/neural net models we've written. It's really good at breaking stuff in subtle ways (like removing L2 regularisation from a VAE while visually it still looks like it's implemented). But personally I still think the juice is worth the squeeze, mainly I find it saves me mental energy I can use elsewhere.