I use my phone every hour of every day throughout the entire year. It's a thing I'm constantly interacting with, both on a physical level (I don't use cases), and on a more experiential level (ie. how the phone behaves, its capabilities, etc).
For something I am always using, spending the relatively few dollars once a year to ensure I'm using something that has the best experience is absolutely worth it to me.
If I'm paying, say, $400 on top of my trade-in/whatever of my previous year's phone, even though I pay it completely upfront, that works out to roughly $30/month if you were to spread it out. Now, granted, for some people, the feeling of having the most capable, most robust device in their hands every hour of every day may not be worth $30/month ... they may be perfectly happy using some previous year's model for some indeterminate length of time saving the $30/month (after they've paid off the phone, however long that takes) and that's obviously an entirely legitimate approach; the same things I'm willing to pay for, others won't be, and vice versa. For me, $30/month every year, and not just some years, is a very reasonable cost for the benefit I get. Heck, I know people that spend more than that per week on Starbucks, and I feel the value of what I get far exceeds the value of what they're getting, and at a fraction of the cost.
And it doesn't matter if it's more than $30/month, the idea is the same.
