I just took a closer look at the details of this offer and nowehere does it say that you must sign up for a T-mobile One plan... in fact if you look at the FAQs it says...
"Do current T-Mobile customers qualify for this promotion?
Yes. Existing customers who are already on an eligible unlimited postpaid plan also qualify for the Google Pixel promotion."
"Who is eligible for this offer?
All existing customers who are on an eligible unlimited postpaid plan or migrate to T-Mobile ONE and new customers who sign up for T-Mobile ONE."
Well since I am already on an unlimited postpaid plan I am fairly sure that this offer will also apply to me. That being said this was just enough of a carrot to finally get me to go ahead and pull the trigger on a pair of Pixels... which with the bill credits figured in will basically be like getting one for free. My wife needed a new phone... and I desperately wanted a Pixel so it just made sense to go ahead and take advantage of this offer. I'll definitely follow up with whether or not we were actually able to get this promotion but I can't see any reason we shouldn't.