I agree. I think the original assumption about "leftover volume/stock" only really applies to the RPi 1. However, if you talk more in terms of ARM ISA, then it certainly applies to every generation of the RPi in that the chosen ISAs are "leftover/last gen licenses" and are presumably less expensive to procure and produce chips from on older design processes. The ARM11/ARMv6 was new in 2001 while the ARMv6Z core in the RPi 1 was new in 2003 with the RPi 1 coming out in 2013. The ARMv8 ISA came out in 2011, with A72 coming out in 2015, while the RPi 4 came out in 2019. [0][1][2][3]

[0] https://en.wikipedia.org/wiki/Raspberry_Pi#Specifications

[1] https://en.wikipedia.org/wiki/ARM11#Overview

[2] https://en.wikipedia.org/wiki/ARM11

[3] https://en.m.wikipedia.org/wiki/Comparison_of_ARMv8-A_cores