1. Apple designed chips are already faster per-core than a top-end MacBook Pro. 2. There's no reason dedicated ASIC modules for specialized tasks (like video rendering) can't be connected by Thunderbolt 3 just as external video/compute modules are now. 3. There's no reason a dedicated x86-64 dongle can't be sold for ARM-powered Macs that need to run software that hasn't been ported yet. It would be a spectacular coup…
2-3. TB3 is a single PCIe x4 link, which is not even enough for a single state-of-the-art SSD. It's certainly not fast enough to connect an entire other computer to, except for certain embarrassing workloads.