If someone were able to compromise the developer account and get a malicious version distributed through the Chrome browser gallery, that would be a huge problem. The kind of thing that has been making headlines with compromised npm modules recently.
Google has reviews in place to prevent malicious extensions from being distributed, but they can't be perfect. We've seen that repeatedly with both Chrome extensions and Android apps.
Every extension with permissions set for "This can read and change site data on all sites" has a huge target on it, and the fewer things using that level of access the better. Ad blocking extensions are an obvious place to look for improvement because they're so popular.
I hope that Google can put a blocking system together that will be able to perform as well as existing solutions without adding any huge security risks, but I also agree that it's problematic that their incentives are to do the exact opposite.
It'd make the Content Blocker API kind of pointless but that'd be safer than letting third parties in.