The extra heat generated by 7.2k drives was brought up in many of the stories that covered the differences.

While they do run normally without any immediate ill effects, as in long enough to run tests to confirm that a 7.2k drive obviously reads, writes and loads data faster than a 5.6k drive, they still generate additional heat.

If no additional cooling is provided... you've already seen what can happen.

Of course, there are other things involved; you said it was a used system so presumably you have no idea how well it was taken care of (cleaning, internal dust clogging cooling system, ventilation of console during use, etc.). I'm inclined to think that the used system you bought may not have been running at 100% in the first place, before you switched to a faster HDD.

Heat has cumulative damage effects unlike electrical surges that can instantly pop an electronic device. I'm curious whether the early signs of failure were there if it was a heat related problem (fans running faster/louder than normally).