Ok i'll break it down.

All Xbox 360s use X-clamps to hold the CPU/GUP heat sinks firmly to the motherboard.

The first 11 million or so Xbox 360s had the largest chips and therefor, the hottest. With heat, the board and it's parts would flex. Even assuming a thorough gaming session weekly would only flex the board 0.1mm a week, that's still 5CM a year. Thus over a year or more, the firm hold of the x-clamp started to become less firm, after a certain amount of slack an xbox 360 will RRoD (1 or 3 lights usually)

This is why some launch models took anywhere from 3 weeks to 3 years to rrod, it was all a gradual erosion. It is also why performing an "X-clamp fix" can fully fix a RRoD 360, because it reaffirms the firm hold (using nuts and bolts) to the mo-bo.

Any way, in 2007 Microsoft released a smaller CPU, then a year later a smaller GPU, then 18 months later an even smaller CPU/GPU hybrid.

The smaller the chips got, the less the internal heat.

The less heat, the less flexing.

The less flexing...the firmer the hold of the X-clamp

This is why RRoD became an ever decreasing issue, partly with the first chip shrink and almost entirely by the time the system chips were both 65nm. These current models are even better, plus as it's a single chip now, theres less chance of either a CPU or GPU acting up, it's all one and the same.

You forgot to mention solder whiskers, and the issues of inexperience with lead free solder causing systemic issues in BGA designs at that time (including the PS3 hence the YLOD issues with the 60GB).  That being said its a good summary that will subsequently be ignored by the large number of Sony fans trolling these boards.  As you say the xclamp design has been fine for over a decade in other appliances, the issue is more to do with manufacturing and the cooling design than anything else.