100% agree with you, what why when i solder the leds to the plate (9 leds from 1st picture) i triple check the thermal compound is not that much or to low, i use usually arctic silver (computer uses) and then again when i screw that heatsink plate to the computer heatsink has arctic silver too.
if the computer heatsink getting that hot doesn't mean the heat transfer is good enough. that why i always try to keep em below 50c degrees.
you certainly understand it well enough but the whole process is quite complicated on a real physics level. Most of the complications have to do w/ not knowing many of the real physical factors such as thermal transfer rate (actual) of the specific heat sink you are using. Even different types of aluminum are different in thermal transfer coefficients...
Of course using fans mitigates a lot of the problems.
Point is if the heat sink temp is recorded the actual temp the diode is at can be 10-30C hotter. That is what plays into the lifespan part. Of course as a DIY and diodes at under $1 each a shorter lifespan is really not important, to a degree...
How to calculate your LED heat sink
As I understand it you have die to base: base to heatsink compound:
Heatsink compound to heatsink and last heatsink to air.
Each having some resistance to heat transfer..
So what ever your heatsink temp is you are pretty much guaranteed your die temp is higher, no matter how you cool it short of ice or refrigeration of course.
Sometimes much higher than you may think..
with fans all you do is make sure that the heatsink doesn't become "saturated" as you would in a free air arrangement and allows more effective transfer of heat..
I ignored for the most part the role in the ambient air plays in this but that really is the more crucial with a contained light or fan-less ones (where ambient could come close to heat sink temp thus slowing greatly any heat transfer) than a more open frame and of course the fact that rooms are rarely above 90 degrees F in my world..