Look at the internet usage difference between iPhone and the rest, I doubt anyone could have predicted this / was prepared.
Perhaps. Perhaps not. Predicting load is a major carrier activity. AT&T has admitted that they used to have lousy tools for it, but claim to have improved.
Reportedly, iPhone owners use less data than other smartphone users. Perhaps partly because some apps were restricted to WiFi. Moreover, carriers like Verizon and Sprint supposedly have carried far more data than ATT for years, due to their having lots of business customers on laptops.
AT&T has said that the iPhone
isn't the most important cause of their problems.
While most people would assume that most of the wireless data traffic growth on AT&T's network comes from the iPhone, AT&T's executives said that isn't the case. De la Vega said that quick-messaging devices are actually driving a significant portion of data usage on the network. - CNet
Just adding 850MHz to major cities also caused a huge problem for AT&T, as suddenly many more people could get a signal inside of buildings, and usage jumped for that reason.
In other words, just adding iPhone users should not, and apparently did not, cause most of the problems people see.
Although certainly the bugs in at least one iOS release (the WCDMA power issue) have caused real network problems. Ironic that it was Apple code, since Jobs originally claimed third party software was a bad idea since it could cause "the West coast network to go down".