Consoles have design lifetimes, and thermals are part of the computation that helps you figure out if your components are going to last as long as the warranty. Obviously, if you run a chip hot, its service life will be reduced.

So companies develop models that tell them how long things like the SOC (main chip with CPU, etc.) will last for a given temperature and use frequency, and whether or not they've got a 1-year design on their hands, or one that will last 20 years.

The model improves as you gain experience with manufacturing, customer issues, and get more testing than you could do before launch. Making the systems too beefy can be expensive, and saving pennies really matters when you're making millions of units.

So I imagine that the folks at Sony determined they could get away with a smaller heatsink and still have an acceptable fleet failure rate, probably a year or two out from the posted warranty.