No it has to be some sort of clear opt-in, at least for media.
We have to remember merchants wont understand a lack of content, empty space, they will only see a broken layout.
They need to see a pseudo working theme in the editor.
A setting at the theme global settings level(Default Image, or Default Content).
Otherwise it starts to go up the platform chain, online-channel preferences level, the custom-data/custom-content level, brand level, or shop level etc.
The placeholder system isn’t perfect but it’s far worse situation for the amount of support problems that can cause from perceived UI breakage from no media rendering at all , without control of the helm.
Yes, it’s a pain for developers to shim but can’t clobber end-users for DX.
There’s a very different side path of this being a tooling issue where devs/designers should be bulk applying their own media refs inside a themes JSON settings files as the method to “override” things “globally”.
But that only works for UI that explicitly consumes a media setting already because the theme schema doesn’t have a placeholder system itself in the cases of default content such as from empty collections etc.