> but so much of what we do is common that if it isn't declarative we're repeating a lot of effort.

We're not only repeating effort, we're also using artisinal approaches to wiring things up. Hand writing handlers is repeated work with low repeatability; there'll be a variety of forms & ways & places people end up writing similar-ish handlers, & creating the data model to pass all the references/targets around.

Being more declarative is not only less work, it also ought be much higher quality, more predictable, to have a lot less vagueries of implementation. It'll make it much easier to comprehend & maintain. Less work, for more repeatable/consistent outcomes.