In a Reddit post, AMD has detailed the reasoning behind its choice of moving to a new socket sTRX4 for the Threadripper 3000 processors. AMD is looking at both near and long-term scalability for the sTRX4 platform, but current Threadripper users feel otherwise.
https://www.notebookcheck.net/AMD-justifies-the-shift-to-sTRX4-socket-for-3rd-Threadrippers-and-promises-long-term-support-but-current-Threadripper-owners-appear-to-be-unconvinced.442727.0.html
It is not justified when AMD can maintain socket compatibility for EPYC and can't do the same for Threadrippers. I believe AMD is doing this to make sure that they continue to have demands for their 12nm wafers from global foundries as per parting agreements... In that way, I believe Ryzen 3000 series will be the last series that will be supported on AM4. It also has a IO die that is built on GF 12nm node. So, for all existing Ryzen 1000,2000 customers the maximum upgrade path will be to Ryzen 3000. This is fine as Ryzen 3000 series is very good in single core and multi core workloads. But, not providing the same for Threadripper owners is a big let down...