While users can choose their primary browser, they have no control over what embedded browsers third party apps use, and so if embedding Chromium becomes popular in third party apps it will unavoidably cut the battery life of many if not most users.
When/if Apple starts allowing third party engines on iOS, I think they should require engines to meet minimum efficiency levels to prevent this issue. As a bonus, it’ll allow savvy laptop users to use community builds of desktop Chromium/Firefox with iOS efficiency compliance flags switched on if they want to extend the battery lives of their laptops by an hour or two.