Gaining customers doesn't mean monetizing, and it's particularly relevant to point out that a channel is under monetized specifically when this channel has a lot of users.
I highly doubt that mandatory monetisation of every channel is a sound economic strategy.
EDIT: While not intentional, that was a bit flippant without extra arguments, my apologies. Also, this is more of a general "abstract" comment; Google operates in a real-world, concrete business context that it has to adapt to, and I am not burdened with knowing the details of that.
Google (the search engine), Gmail, and really almost everything in Google's ecosystem are essentially freemium apps since before the words "freemium" and "app" were coined, let alone combined.
You can use Google for free, but if you're trying to sell something you can use it's advertising.
You can use Gmail, and other the services the implicit Google account it gives, for free, but for the niche use cases there are things like Gmail business, or extra disk space on Google Drive once you run out of space to upload those big photos and videos from your phone, and so on.
What Google has expertly done is position itself as a public utility of the internet, despite being a private company. Whatever they are planning to do, I think that if they mess with that too much they risk a lot.
That is what I believe adamsvystun's point was: compared to most other monetisation options, ensuring that they keep this "public utility" position in the public eye is the most lucrative form of long-term monetisation for Google.
EDIT2: It gets worse the more I think about this: messing with email is especially risky, because email itself is tied very strongly to one's internet identity. It is the passport of the web. Think of all the "sign up/log in with" options: email is always the fallback option. So not messing that up is incredibly important to Google.
I don't quite buy into the public utility thing. Any Google product has competitors. The reason people use Google products is because Google advertises its new products on its products people already use massively. I don't think people actually see it as a public utility and are unaware of the competition. When you're in use-case A and suddenly you have a link from the same service to subscribe to use-case B, there's some effort required if you want to study the alternatives, and people prefer the easy way. Google crawls its way from one oligopoly to another this way.
