They help streamlining the control flow, and take away control code while providing you with an incredibly cool developer experience.

You’re allowed to blame us for a terrible developer experience in Trailblazer 2.0. It’s been quite painful to find out which step caused an exception. However, don’t look back in anger! We’ve spent a lot of time on working out a beautiful way for both tracing and debugging Trailblazer activities in 2.1.

Finally, you can use fields_for in order to create/edit associated fields and the suffix _list (like profile_list below) to choose an existing associated record.

balancing developer and user experience

hygienic from an app-developer standpoint

I have no familiarity with Svelte internals, so much of your talk about what they would do eludes me. I'm just concerned with developer ergonomics

In general, my focus has shifted from optimization to DX. Partly because Svelte does a lot of the heavy lifting. For things that can be optimized on a need-to basis, I would rarely sacrifice DX.