Wouldn't it actually depend on when you buy the phone? If I buy a Pixel in one year (just before the "Pixel 2" is released), presumably I will still receive the two years of updates...
Alternatively, if I buy the Pixel on it's release date, is it safe to assume I will receive 3 years of major updates + 1 year of security updates?
Alternatively, if I buy the Pixel on it's release date, is it safe to assume I will receive 3 years of major updates + 1 year of security updates?