A Nexus user doesn't have to use Google services and an iPhone user can use mostly Google services.
Your statement makes it sound like Google only makes money off of advertising to Nexus users when they still make a ton of money from iPhone users. Maybe more.
The Nexus 6P is a hell of a phone for the price.
It is pretty dam hard to not use Google services on an Nexus phone. SMS is baked into Hangouts. Nearly everything is centered around Google Now. Even if you don't use gmail for your email, using the default email/calendar/contacts app for any email/services means data is extracted from them.
It is certainly possible to not use Google services on a nexus phone, but I think a very very small percent of their Nexus-using population would do that, as it would require jumping through some hoops.
Certainly Google has it's hands in the iPhone as well. However, other than the fact that default search in Safari is Google, everything else Google on the iPhone is opt-in and totally optional. Google can't index my work email, my SMS conversations (let alone imessage), my contacts list, my notes, my calendar, Google doesn't know what apps I'm using, etc. Even if I do choose to sign-in to Google on the iPhone, it doesn't get anything extra than what it already has (i.e., signing into Google on iPhone doesn't let Google index my contacts that are stored in iCloud).
Again, I know this always sounds like I'm implying Google is evil or something. I don't mean to. I understand the quid pro quo with Google, and I choose to accept the deal
sometimes. I see it as a subsidy - either phone is $750 bucks. With Apple, you pay full price. With Google, $200-300 is taken off the top by Google in exchange for the ability to mine a whole bunch of data about you. The choice is subjective, one is the right answer for some, and not others.
I agree the Nexus 6P is a hell of a phone for the dollar price. There is a lot more than just your dollars that Google is receiving as compensation when you use that phone.