There are clear warnings with the iPhone about temperature range and the risk of condensation. As far as a seven year old phone being less susceptible to water, well yes, of course. It didn't have a fancy touchscreen, large amounts of flash memory storage, wifi, GPS…
It's not really so much to do with the technology in it. It's to do with the obsession with making devices as slim as possible. Which means things like having a very minimal headphone socket which is basically a hole directly to the circuit board (unlike older, larger phones), a gigantic dock connector socket, and generally being very poorly sealed to save space.
If you open up an old fat nokia phone, and a modern phone, it is incredible the difference between how packed up the compenents are compared to the older one.
I don't think there's any conspiracy going on here, it is just that phone companies make the major selling point be that it is 0.5mm thinner than the last one, yet has all the same stuff in side, and something has to give.
The only bad thing in this story is that Apple have always been a bit flaky about warranties or manufacturing problems with their stuff*, but like someone said above, due to the desirability, and the way their fans want the latest version, they do probably have a higher than average level of phones 'breaking' just as a new one comes out.
Joe
*the old Macbooks were pretty dodgy