People forget that there are plenty of ARM systems that have PCIe slots/lanes. Even my synology NAS with an ARM chip has a PCIe slot. People need to relax. I am pretty sure they will add PCIe slots in the newest Mac Pros. The only thing I am not sure about is the memory situation since Apple Silicon has the RAM baked into the SoC. However there is already DDR5/DDR6 available via Ram Sticks, maybe they'll figure it out.