Uh-huh...
And the lever is???
And the fulcrum is???
Just because you can quote someone who noticed the water level in a bath rises when they got in it, doesn't make you appear any smarter. It's just annoying, and not even relevant.
Anyways, seeing as I actually have studied engineering and physics, I did some moment calcs. If the iPhone 6+ can take 90 lbf in its centre, but fails just below the volume button, that means if the load was applied AT the weak point, it could still take around 67.5 lbf. But the critical part is that would correspond to a 45 lbf reaction force at the top edge of the device. At an absolute minimum.
Is that force possible to apply when it's in someone's back pocket of a pair of skinny jeans, and they sit on a hard chair? Possibly.
How about in someone's front pocket? Probably not without a lot of pain, as it would be 20 kilograms of force applied to a very small area of your leg. i.e., Ouch Time.
My subjective opinion is that it is not reasonable to expect Apple to design their phones to be willingly sat on. Despite owning many pairs of jeans with rear pockets, I don't think I have ever used them, because i don't want to damage the things I own by sitting on them, and it would be uncomfortable anyway.
And no, I don't own an iPhone 6+, but I do own a HTC.