So - a bit on TJunctions. – First of all, regarding durability, when Intel sets a maximum allowed temperature, they set it with a "safe margin". This means that if you run at 100C all the time, nonestop, the CPU is still expected to last a "normal product lifetime". This is not to say it won't last longer at lower temperatures, because the electron pathways will wear down slower, but it's likely something else will fail before the CPU will anyway.
Second, the throttling concern. First off, even if the i7 does throttle under full load, it will still produce better results than the i5 at not-full load tasks. But based on experience with my current iMac (albeit a 2014 i7), it won't throttle significantly. My CPU is rated at 4GHz for all cores at once, and turbo at... What was it? 4.4? Anyway, during heavy CPU tasks, where all cores are active (including hyper threaded ones), it usually runs at around 4.1GHz, which is actually above spec. Blast the GPU at the same time though, and it'll run between 3,7 and 4.2 (swinging up and down). But it's hyper threaded and have more cache than the i5, so at the same frequency, it'll still outperform the i5.
And lastly, yes. Apple has made computers with insufficient cooling for the hardware inside. Their computers are built based around usage, not worst case scenarios. This means that if you blast the CPU, GPU and all parts of the system 100% at the same time, a workload that you're basically never going to run into under real circumstances - the cooling will be very insufficient, and will either result in severe throttling or in the worst case, the computer going to sleep on you or shutting down entirely to save the components (a nifty little feature that again increases reliability). But again, this is in very forced scenarios that won't happen naturally, and if they do, will only happen for a single second or so, which the cooling will handle.
