I'd argue that Google implemented it because they don't want their product to be implicated in a high-profile attack; if someone disappeared because their Gmail

I'd argue that Google implemented it because they don't want their product to be implicated in a high-profile attack; if someone disappeared because their Gmail credentials were phished, it could easily blow back on Google and contribute to a perception that Google services are fundamentally insecure. It might be a harder sell to implement E2E crypto, although perhaps the same argument might apply some day. The notifications are probably just low-hanging fruit.