I think you're misinterpreting what Tjunction actually is. 100 °C is not the boiling point for transistors at which they magically disintegrate and break for good. Depending on source material some transistors can operate at much higher temperatures while others might stop working at 60 °C or less. Intel specifies that 100 °C is a safe temperature to operate at. Above that the CPU will start to thermal throttle in order to protect its integrated circuits not necessarily from potential damage but in order to ensure reliable operation. In the olden days of yore, long before CPUs started to thermal throttle, they simply froze and stopped working until a reset was performed. Thus, running a CPU at 99.7 °C is absolutely fine and nothing to worry about.
Honestly, what difference does it make to you whether a piece of technology inside a metal case hidden behind a glas screen is running at 99.7 °C, 85.2 °C, or 66.9 °C? At the same time, you insist on quiet operation at full load. You can't have your cake and eat it to. No matter how expensive an iMac is, whether it's $500, $5,000, or $50,000 it cannot overcome the laws of physics or, more specifically in this particular case, thermodynamics.
