If one is going to buy the phone anyway, payment plans with 0% interest make a lot of sense (from the buyer's perspective).
I suspect that for a lot of people, without the payment plan they would not buy anyway since they can't afford to buy these outrageously expensive devices outright.
To everyone their own. I very much respect your approach. But when I'm upgrading I don't want last year's model. My phone is the single most used "possession" in my life and after using one for two years, I like to treat myself.
The thing is if you always sit a year or 2 behind the cutting edge you still get to treat yourself because every upgrade is a huge jump in technology from what you had before. Unless you are comparing yourself to others it really doesn't matter because you still get something better and new to you.
I have found that it is insanely cheap to just buy last years phone second hand. I picked up a pixel 2 recently for $300 AUD when it was about $1000 the year before.