There's a comment on the Chromium blog [1] (via magicalist's comment [2]) that nails this: if security is all they cared about, a signed certificate is all that's necessary. Knowing that option exists and persisting with their store-only approach means they must have an ulterior motive of some sort, most likely control and money. Ad blockers that target Google Ads will be no more, as well as anything else that the us…
Of course the problem with that route is preventing any unwanted-extension installer from maliciously enabling that option itself.