This is actually a big discussion point within Apple. In truth, they have wanted to stop using this particular phrase a couple of year ago (iPhone 7). However, Apple Legal and Apple Investor Relations department maintain that Tim (or whoever is on stage at the time) must say this phrase.
Otherwise, it would be Fiscal Malpractice and the shareholders would sue. Obviously this is due to both the Financial Media and the Tech Media parsing every single word that comes from Cupertino for clues about the companies products and direction.
They had an opportunity last year to not use the phrase, since obviously the iPhone X was not the best iPhone ever. But alas, Tim chickened out. So now we're stuck with another few years.