Uhhh...Apple has been ignoring the Professional community for years. Yes they have released a couple systems to not completely abandon them but they are clearly more focused on Phones, tablets, and laptops which get refreshed yearly, vs professional computers like the Mac Pro line which took 6 years to see an upgrade. Maybe that changes with the move to Silicon but if recent history is any indication I wouldn't bet on it.
As far as eGPUs, we can say with a lot of certainty the current eGPUs that are built around Intel chipsets won't work as is.
Best case scenario is we have something similar to the PowerPC days where we can "flash" an Intel card to make it work with silicon.
Worst case scenario we have a situation where the architecture is too different and the eGPU is some sort of Breakaway box that is built solely for silicon systems which means there will only be a couple expensive options because 3rd parties are not going to line up to build something for such a small community.
Worst worst case scenario, no eGPUs with silicon.
I guess there is another best best case scenario in that silicon is so powerful and awesome eGPUs aren't needed.