Linus is smart enough to do his own calculations (if he had the time). He had his reasons and made his choice, much of our commenting have been a waste of time (here and on other forums).
If you really want an explanation/speculation ...
1) Compiling with multiple threads has diminishing returns when increasing CPU cores as you hit portions of the process waiting for 1 or 2 threads left to finish. Disk I/O can now keep up with 3x CPU performance when comparing NVMe SSD to SATA SSD. The whole system being new, it's hard to tell what impact each component has on the result. Compiling can involve lots of branch instructions, cache size and small IO.
2) Linus spends most of the year with emails and looking at git/code where it doesn't need a lot of CPU. Then it comes to the merge window and then for several days it's lot's of compiling and testing making use of the extra CPU cores.
3) Linus doesn't upgrade every 2 years, more like 4 years. Intel have been recycling 14nm designs for about 5 years and making minor improvements then offered factory overclocks. After getting the kernel compile time from over 65secs down to about 25secs, an extra 5secs is not as significant and not worth spending $1000's more. His computer at home is probably sitting idle while travelling, phone calls and during meetings.
4) Linus was frustrated by Intel's response to Spectre and Meltdown and other security vulnerabilities may have influenced the decision. Linus in general doesn't like bugs, poor security, slow responses, lack of information, crude fixes that greatly impact performance (eg. disable HT they said) but I'm not pointing the finger at just Intel, has happened to many. Linus was not rushing to AMD when past AMD based systems had MB/BIOS bugs and poor/late drivers for Linux (CPU/APU/GPU). Now AMD systems and software have proved stable, mature and worth the chance.
5) A 32 core AMD is maybe 2.5x to 3x the performance of an Intel 10 or 8-core from the last 4 years ago. The 32 core AMD is double the compile performance of Intel_14_core, 1.7x Intel_18_cores while saving 30% power at peak loads. But, the total system power used by the AMD TR is slightly more than Intel under low loads. Both get very hot when using all-core turbo to the max (AMD = less peak W per core).
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