> everything to lose
Really? Good luck suing them if they decide to sell your data. They certainly won't lose much revenue from fleeing customers if they think up a creative new way to monetize your data with "select business partners"; Google (and Facebook et al) spent the last decade entrenching themselves infrastructure for far too many people. They have far too much power and inertia to lose much in the short or medium term.
> trust them
Privacy policies change. Even if Google has good intentions about protecting your data today, you're gambling that those intentions will not change in the future. You don't know who will be hired/fired at Google in the future, nor do you know how the current (or any) management will react should the company have a particularly unfortunate run of bad finances or other troubles. Never-mind that humans often act irrationally for stupid reasons so any prediction about future behavior has to have a huge error bar.
However, that isn't the big problem with trusting Google to not share your pattern-of-life[1] with a 3rd party: you're assuming it will be Google's choice, or that they will even have any de facto influence over the long-term fate of your personal information. Your trust doesn't make Google infallible; the best security teams can only make hacks less likely. Warrants, legislation, and quasi-legal-but-hard-to-ignore orders from governments happen. Prism (and other mass surveillance programs) still exist. Concentrating valuable data at one location makes it more valuable, so the scope of potential threats to your data increases as more data is collected.
The world is not as just[2]. Trusting that your data will magically stay safe at Google forever - or even just the near future - is only possible if you first pretend that Googles security is and always will be perfect, that the programs Snowden/Drake/Binney/etc warned us about never existed, that no current or future Google employee will ever become disgruntled (or crazy), and probably many more potential threats that haven't been invented yet.
[1] or mundane data like your email
[2] https://en.wikipedia.org/wiki/Just-world_hypothesis