I read the parent post as asserting that the value of ads placed on youtube is not as large as the incremental value to non-youtube ads (which is a much, much bigger market) from better targeting enabled by youtube data.

I don't know if that's true - youtube ad revenue seems to be ~10% of total google ad revenue; so for that to be true youtube data would have to provide 10% revenue boost to existing ads over all the other (many!) data sources that google has, and I'm not certain if it's realistic.

if it wasn't google who bought youtube, the viewership data would only have been useful for the ads on youtube. But because it's google, they can leverage this data on the entire google platform, and thus derive more value from it. This means they can afford to make less money on youtube and still have it be valuable. That is why google bought it and not microsoft (or another non-advertising company).

youtube's revenue remains undisclosed in google's financial statements (see https://www.nytimes.com/2019/07/24/technology/youtube-financ...). The $15-25 billion is merely a guess.

The fact that google chose not to reveal youtube's financials is stark evidence that it's financials aren't great (otherwise, disclosing it should lead to better stock prices for google!).