> The idea behind that originally was that all of these other projects were going to become so significant that it wouldn't make sense to have them or their management coupled with Google.

This isn't entirely true. A major consideration was fear of anti-trust litigation. If all of these are the same company/orgs/departments, then you could reasonably say that this "search company" is far too powerful. If there's a search company and a youtube company and a self-driving car company (etc.) then you can make a (specious) argument that you're not vertically and horizontally a monopoly.