You're wrong. I also do this all the time and if my phone broke I'd be outraged because every phone built before the iphone 6 can handle that kind of stress.
My ex-wife always sat with her 3 in her back jeans pocket because fashion designers refuse to put sensible front pockets on women's jeans. The 3's case began to crack at the top around the power button after a while. And this is a girl sitting on her phone - not some 200lbs guy.
And I certainly wouldn't sit on my nexus 5 or the SGS4 that it replaced.
Eh. I (until recently) weighed around 220lbs. I would not have been surprised if any phone I sat on [0] broke or was otherwise damaged. Especially with the way that anything in a back pocket gets flexed. It's not just the weight placed on it, it's the tension from conforming to the shape of your body along with the pants. I guess if I had no ass, or weighed much less, or like some coworkers wear pants so low that the back pocket is actually behind the thigh it might not be a big deal.
[0] This also depends on the seat. My couch at home is very cushy, it'd be hard to break it there. But my kitchen table chair is little more than a slat of wood cut to a comfortable shape.
I'm 185 pounds, and I'm absolutely confident that if I ever placed any of my iPhones in my back jeans pocket and sat on it, it would be destroyed in under a month.
It has a glass screen, that alone should make it obvious that you shouldn't sit on it.
Older iphones were recently rated by Consumer Report to be about twice as resistant to bending from pressure and might well have endured this with ease. Also, the gorilla glass screen may have been less scratch resistant (though more than good enough), but it is less prone to shattering too.
So once people may have fixed certain expectations about the durability of an iPhone. But those will have to be revised with the new models.
The phone is the outlier, not the use case.