I think Datastar has the better approach here with making OOB the default. I suspect HTMX's non-OOB default makes more sense for very simple requirements where you simply replace the part of the DOM from which the action was triggered. But personally, situations where OOB is necessary is more typical.

Interestingly, elements sent via the HTMX websocket extension [1] do use OOB by default.

[1]: https://htmx.org/extensions/ws/