Thanks for filing this task, @Daimona!

It is encouraging to hear that the migration is considered a small task.

If we were to do this work, I wonder what could potentially be unlocked - for example, would it be much easier for us to take on work such as T353200, T385606, or T347198?

In T398356#10996886, @ifried wrote:

If we were to do this work, I wonder what could potentially be unlocked - for example, would it be much easier for us to take on work such as T353200, T385606, or T347198?

AIUI, at least in its current state, the benefits would be on the technical side only. For example, quoting from https://www.mediawiki.org/wiki/Manual:Notifications#Benefits:

For extensions that already use Echo, it allows omitting checks for Echo being installed, and provides a more type-safe interface than Event::create(). We hope that the middleware system will also prove to be a more reliable way of overriding other extensions' notifications than the current variety of hooks defined in Echo.

I don't think any of this would be noticeable by users. It might make user-visible changes easier to implement later, but I don't see any mention to those for the time being.