Siri pulls from Messages for those things as well, and will pull spelling and other suggestions out of Safari browsing.

Because... Google's business model. Your ability to see your purchase history on a Google page is neat, but it seems a safe bet that the real reason they want your purchase history is because their profile about you as a set of eyeballs becomes more valuable allowing them to charge higher rates on ads. Notice that they only claimed to have stopped scraping gmail for ad targeting purposes, not that they stopped scraping period.

Gmail's primary UI is a web browser. How... how do you think that _actually_ might work?

Exactly. Google is saying they "need the cloud" to make the prediction/action. They also require as grist for their mill: my data.

I would much prefer some basic heuristics executed on-device using my data in a secured processing state. The hueristic-building could be in the cloud, but on datasets other than mine, and let me tweak those dials!

Hell, I'd even pay for this feature.

It's very unlikely that Google employees are going to be representative of Google's users as a whole (just look at the startups building niche products for Bay Area techies). Ditto if you only looked at data of users that opted in (maybe they're highly unrepresentative).

That means the heuristics/models that get built would be a worse fit for you, as a user, although it sounds like that's a tradeoff that you'd like.