I want to belive that this is because bitcoin mining has hit the pinnacle of Moore's Law, which states that processing density doubles every 18 months, there are no more huge jumps left.
the failure of some people who think that bitcoin asics are at their limit. is a failure of understanding all the variables
its not just about chip NM size of transistors. other variables can be tweaked far beyond the NM measure.
for instance even if we say that transistors are stuck at 7nm.
(its less, but lets use large numbers for easy demo)
what can be changed is the sensitivity of the transistors.
EG imagine transistors 'switch' based on a receipt of 0.1 volt. new 7nm transistors could "switch" at 0.05volt. meaning the 'density' of the transistor has not changed. but less overall voltage is needed. and less voltage means less heat and so the gaps in the chip between transistors and thus the gaps on the circuit board between chips and thus the gaps between different circuit boards can be pushed tighter together, due to less heat generation....
.. this then means either less electric used per hash if same hash per second.. thus cheaper..
or due to tighter circuit boards inside the asic, fit more circuits and double up hashrate. even when the nm size of transistors has not changed
.
or looking at the macro level.. although the network does have ~1.5mill asics running at the same time.. nothing stops people just buying more asics.