Honestly, this is a really cool feature from a security standpoint. It prevents a lot of different attack vectors that are particularly hard to mitigate.
As for re-use: My first thought was that AMD should have allowed for users to disable this feature (with a motherboard switch or jumper), but even this re-introduces issues if the supply chain is compromised. The ability to enable/disable PSB would likely have to be handled on the CPU itself to not compromise the security it provides. That isn't easy to do in an end-user-configurable way. Security without any compromises is challenging, unfortunately.
I understand the e-waste / re-usability argument, but I think it's overblown (at least somewhat, anyway). If the second-hand market ends up being flush with non-vendor-locked systems and vendor-locked CPUs, where did all the systems the vendor-locked CPUs came from end up? I'd be interested in knowing what percentage of hardware even makes it onto the second-hand market to begin with, honestly. I'd bet the number would be surprisingly lower than we'd expect or hope for. In strict security applications, entire servers usually get shredded after they're past their usable life (per various regulations or ease of compliance); this would have zero impact on that (fairly large) sector to begin with.
