Not sure what the discussion is about anymore ,
Carfax83 - your point is that multithread is important ? and
nxre point is that single thread is king ? welp both are important and some are more then other depending on your usecase , this is why we have a market for threadripper and a 5600x (even when ignoring price) , one is better then the other in games for example , but the other one will run your export faster , note if games would run better on threadripper , you will see it in high end no budget gaming machines (and that day might happen someday).
But in the current world of consumer electronics and the usecase they generally employ , single thread performance is the most important metric assuming you dont sell a 2 core machines , as nxre correctly pointed that out , Apple are not selling a 1 core CPU , and there current offering has enough multithread capabilities to do the tasks they deem important very well.
Adding more cores to a MBA will make it run usecases that are so niche for the folks who bought that computer and will hurt the things this machine tries to achieve that it wont be worth it in the overall scheme of things.
"which is that performance critical applications have practically all been optimized for multicore/multithreading and SIMD, which had massive impacts. "
What is the point you are trying to make ? i am so lost in this conversation not sure i can grasp it anymore - what are those critical performance applications you expect a 1000$ fanless laptop to compete in? why would the correct tradeoff for this machine would be to put more cores ? do you think Apple are not capable of putting more cores in their SoC`s ?
Apple couldve designed a 16 big cores CPU and stuff it into a MBA , that machine would have beaten everything in the laptop market and most of the desktop in multithread performance , but the machine would be useless as it would drain too much battery and wont be able to run fanless , gggrrrrr i feel like i am missing the point big time here as i type my reply , sorry about that , there is no way i understood the discussion here , sorry in advance

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Have a good weekend guys , should drink my coffee before replying i guess , this one is all over the place.