..except for say, renting the information to "partners" for linking with offline purchases
Google doesn't do that. Rent, sell, donate, whatever. If you have some evidence to the contrary (e.g. public financial filings?), I'd be interested in seeing it. So would the FTC, actually, since AFAICT it would be a violation of Google's consent decree.
or if you switch browsers or somehow the cookie gets removed or you switch to a private browser window
I'm not entirely sure what you mean here, unless perhaps you're talking about losing your opt-out cookie? If that's what you mean, Google provides browser extensions that ensure that never happens.
Google doesn't only derive value from the information they gather about you by displaying you targeted online ads.
Yep, pretty much, that's it. Unless you're paying for Google services or buying Google hardware, online advertising is Google's revenue model. If you have some evidence to the contrary, I'd be interested in seeing it.
There are reasons why every ad network offering an 'opt-out' only stop displaying you targeted ads while it is in effect.
Again, I'm not sure what you mean here. Are you saying that if you stop opting out from targeted ads you start seeing targeted ads? That seems pretty obvious to me.
And none of them are for your benefit.
None of what are for my benefit? The ads? If that's what you mean, I beg to differ. Most ads are useless to me, I agree, but it does happen from time to time that I see one that's useful. Even more importantly, those ads are how the sites that I like get funded, so they benefit me very directly.
(In my particular case, Google ads also pay most of my salary. But I felt the same about all of this before I joined Google so I honestly don't think that affects my opinions much.)