Well frankly I think my "they might read my chat in finch" is a stretch, but no, I don't think that assuming google's goal is to stretch their tentacular roots as deeply as possible into the fertile, loamy soil of my private information is "slippery slope conjecture with no basis in reality." To the contrary, as you probably know, gathering personal information on users is the primary purpose of all of google's unpaid products and the heart of their business model. Would you dispute this fact?
So they are a targeted-ad company whose value is to a great extent commensurate with the amount of information they have on users (another uncontroversial statement, I hope). More info = more value, and you think it's "conjecture" to speculate that they might push further and further into your experience on their platform?
This is the crux of it for me: I've been installing desktop applications and browser extensions for years, and I've never needed to tell an advertiser and the biggest tracker / data snooper on the 'net what my email address. Now all the sudden I do, and there's no (clear) method to opt out, and I'm OK with this because why? I'm willing to trade my data for services when the services are valuable (I use gmail & facebook), but this looks like "give us more info, just cuz" with no value prop. Sure you can sync but I don't want to and there's no reason you should be forced to sign in; that is 100% about data gathering (otherwise there'd be a "forgo sync" option).
IP address and browser fingerprint is not the same as "link everything explicitly to my google account," by the way.