In April, Apple said it has sold 20 million Apple TVs total, and the current numbers seem to point to 8-10 million a year. If we assume their engagement is similar to those of Chromecast users (3 times a week), that works out to between 1,248,000,000 and 1,560,000,000 “uses” of the product in the same time period.
That's a great number and good for Apple. Apple suggests sold numbers because as a company, their business is hardware sales and it is their indication to the public markets.
Google is a services company and the business is in usage. I am glad Google chose sessions as a start, but having x-day actives would have been the right number to share.
Corporations will continue to report the numbers that corporations care about. IOW, every corporation is playing the same game.
FWIW, I use a chromecast and it is the only HDMI port that is used at least once a day and the Apple TV was the only product that I purchased and returned back as I couldn't fit my media consumption into the Apple way.
Note: Apple TV has been available for purchase at much greater than $35 since 2007.