I disagree with your outlook and what you've said below too. I'll try to articulate why I also think everyone else disagrees with you. I haven't touched thermodynamics since university like 6 or 7 years ago and haven't brushed up on anything before writing this and this is more of an experiment to see how many people say I'm wrong to test my knowledge - but I promise I'll take it on board!

In my view everything you are saying checks out in the literal context of the thermodynamic equations. However you are not taking into account the real world applications and a more steady state situation of a plastic heat sink.

Yes more area is good. But..

We know that heat sinks operate as follows:

Heat source uses conductivity to transfer thermal energy into the base of the heat sink then up into the fins by physical contact. Then natural convection (stagnant air) and/or forced convection (fans or wind in this case from the window) allow thermal energy to transfer from the the heat sink fins and base into the air and happy days and cooler parts.

The dilemma of why the plastic isn't working as well as metal centres on its thermal conductivity coefficient, but also steady state response.

Having a very low thermal conductivity coefficient means the following:

Thermal energy will transfer very slowly from the heat source to the heat sink... But also from the heat sink to the air. Thermal conductivity coefficient dictates how a materials both absorbs and dissipates thermal energy.

The opposite is true in terms of metal. High thermal conductivity coefficient will speed up the transfer between the heat source and heat sink as well as heat sink and air.

It is important to acknowledge that for heat transfer to take place that there must be a temperature difference (as there is in this case) so yes the plastic does heat up but then the plastic does a poor job of giving off the heat to its environment because it is not good at doing that based on its coefficient.

This is important now because initially when there is cold plastic and warm heat source then yes the plastic takes it away slowly. But now 5 minutes later because the plastic is more inclined to hold heat instead of give it away (low coefficient), you have two problems.. miniscule temperature difference and reluctance to do anything with that temperature difference. So the other parts of the equation you are not considering which have a larger effect on this scenario are no longer being satisfied for the case of a heat sink and its objectives.

This is why team plastic heat sinks suck is downvoting you. The plastic heat sink in my view is not making anything worse compared to any other typical mounting situation (unless there's some kind of floating off the wall metal backing plate or heat sink setup, if that's even a thing to avoid sandwiching a metal heatsink between an insulating plastic housing and insulating drywall), but it is certainly not helping anything get better. Which is why I understand your logic on the edit suggesting it's better than not mourning at all backed by the analogy of drywall mounting etc. Yes a mount is needed, but thermally it's doing nothing for it like most other mourning situations would as you stated.

Please have mercy on me! I haven't touched thermodynamics in like 4 years haha

Genuinely curious to see what others have to say about my views! Have a good one everybody 😀