This bit is a little disingenuous.
Starting with an accusation. I guess that strengthens your argument. Thanks for that.
Google sharing their users' data is like giving away your secret recipe. They need to keep it close to their chest so that their competitors can't target ads as well as they do.
Google gets paid to disseminate or facilitate the dissemination of its users' information. It doesn't serve them to sit on the data rather than starting to earn money immediately.
Some of the following is a bit of supposition, but not disingenuous.
They don't open up their data stores to other companies since they would lose the business advantage of being able to continue to sell information derived from it. Requesting that information to be deleted does allow a user to temporarily stop the bleeding, but the damage is already done. Data brokers probably have access to the information within minutes of it being collected. Certainly the damage gets worse over time. The more data that's collected and correlated, the more information about a particular individual can be deduced. And it might not be Google's data that is the risk at all. Their tools and use of their products facilitate other companies in collecting your data. I gave an example in an earlier post of how Google search facilitated Facebook in collecting information about me.
I can think of two possible benefits a user gains from deleting their data: if Google develops new techniques to monetize user data, they'd have less ammunition without the full history of a particular user and it might be harder to correlate a user's information from before the deletion with the information from after it.
Regarding that latter, we shouldn't be so naive. As soon as data starts getting collected again, the aggregators just start soaking it up again. Big data techniques would probably make it trivial to correlate the data from before and after the deletion with a very high probability.
To allow a user to prevent the damage, Google would have to provide the same kind of toggles that Apple provides which stops the data from being collected in the first place.
Since I am guessing a bit, I do listen to responses to evolve my opinion. But, I have such a long way to go to feel comfortable using Google's AI.