> 1) you dont care about keeping UI state on the parts that are replaced (incl any stateful children of the UI element that happen to be there in your DOM structure), and 2) when you dont have to update the app in any other places when your state changes

Htmx does have tools for both of these cases. Out of the box, htmx throws out state, but there are plugins such as morphdom-swap for merging in the new DOM fragment into the old while keeping state. I have some client-only state that holds references to DOM elements in Javascript, and by default, yes, htmx breaks all those references as those elements no longer exist. Link in morphdom-swap, and my references live on across reloads, even across attribute and content changes.

And for #2, htmx also allows you to swap in elements that are not the target element, just by specifying that that's what you want.

IMO these are pretty basic tools of htmx. Like you said, without them about the most complex thing you can create is a to-do list, and sometimes not even that.