Unless they sell the Pixel directly from Sprint, I wouldn't hold my breath. I bought my N6 through Sprint and bought a SIM card for my unlocked N6P last year, and have had a flurry of problems with Sprint since.
First, the process of purchasing a SIM card for the 6P was arduous and required lots of digging on my part. The Sprint salespeople were remarkably uninformed about which SIM cards would work for the device. (And no offense to them either - they can't be expected to know the exact SIM that will work with every single unlocked phone.)
Second, call quality and service on the N6P has been nothing short of a disaster. Sprint's reliability is already so-so compared to Verizon in most areas (no matter what their commercials tell me). But comparing the signal strength in my NYC apartment (never below -105 dBm) with that of a friend's Sprint Galaxy S5 in the same apartment (-93 dBm) demonstrated that there is poor network optimization for this phone in particular. My N6 suffered from a glitch in which turning on wi-fi would toggle off service for the phone, unless I turned off LTE. You can imagine the scenario that happened over and over again: when wi-fi was on, receiving calls would immediately go to voicemail. I have not heard similar tales from N6/N6P users on GSM carriers.
Third, basic features that work on other phones - see wi-fi calling - did not work on the N6P, despite wi-fi calling being a built-in feature on the device.
My hope is that Google sells the Pixel directly from carriers and performs quality assurance to make sure that the Pixel is optimized for every U.S. carrier. Another direct-from-Google fiasco would make me nervous about using the Pixel on Sprint.