IIRC, and it's been 15 years, for the last transition they sold prototype towers that could only be purchased and operated by folks with an active and current A

IIRC, and it's been 15 years, for the last transition they sold prototype towers that could only be purchased and operated by folks with an active and current Apple Developer account, and did not make consumer hardware available for 15 months after the June announcement. So at the first WWDC the developers get a one year head start to work with Apple to get everything ready for the new platform, and then at the second WWDC Apple finalizes the launch plans and sets the launch dates for new hardware. It seems likely that Intel editions of the 16" MacBook Pro and the Mac Pro will continue to be sold after September 2021, due to hardware DRM dongle issues — and it's likely that they'll keep around an Intel Mac Mini as well. Given that ARM is now a datacenter option, it's probably time for the JVM ecosystem to stop being Intel-only in any case, but historical evidence suggests you have at least 15 months before general developers have any hope of buying one of these.