You mentioned "initial downclocking effects", yet (for posterity) I want to emphasize that in 2020 Ice Lake (Sunny Cove core) and later Intel processors, the downclocking is really a nothingburger. The fusing off debacle in desktop CPU families like Alder Lake you mentioned definitely killed the momentum though.

I'm not sure why OS kernels couldn't have become partners in CPU capability queries (where a program starting execution could request a CPU core with 'X' such as AVX-512F, for example) -- but without that the whole P-core/E-core hybrid concept was DOA for capabilities which were not least-common denominators. If I had to guess, marketing got ahead of engineering and testing on that one.