People are keeping their phones for longer and longer. This makes it as attractive as possible to pay for a new phone at a more frequent rate.

The bet here is that Apple would much rather you spend ~$35/month perpetually than $1200 every 4 years.

This is a 'good deal' if you're already someone who upgrades their phone frequently, but I would imagine this would end up getting a whole bunch of people to pay more money in the long run to Apple.