Gruber gave a good argument as to why the numbers must remain. Unlike iPads, Macs, etc, Apple still sell older models (and if Apple don't, often authorised resellers still will - there are still lots of places to buy new iPhone XSes, even though they've been discontinued by Apple). Therefore there must be a way to differentiate them by name.
Perhaps the other solution is to call it something like iPhone (2019), but that's probably undesirable from a marketing perspective as it will seem outdated very quickly.
Though I do think 'Pro Max' is a silly suffix. I never even really liked 'Plus', spelling it out seemed unnecessary when they could've just used the '+' character.