It’s never “fast enough.” PCI-E motherboards are actually out now. In three years we will have either PCI-E 4.0 or 5.0 as a standard.
And you’re right—PCIe 3.0 cards out today have plenty of performance for today’s applications. That why you would get a PCIe 3.0 card today. But it doesn’t matter if it’s fixed because you aren’t going to upgrade it.
Actually that's not true. When PCIe 3.0 was introduced, it took quite some time before the (still marginal) performance improvements materialized. We're talking about a few percent at most in games and benchmarks (many are just neutral) between PCIe 2.0 and 3.0, simply because it's not a bottleneck. For not overly data-intensive GPGPU applications (e.g. video things) I reckon that's the case as well. Yes, sure, there are applications you can run on a GPU where PCIe is a bottleneck. PCIe bandwidth benchmarks, for example.
Don't expect PCIe 4.0 consumer-ish (i.e. what this is about) GPUs soon (2019 earliest). It's wholly unclear whether you'll see any performance improvements at all from that — just look at the bus utilization in games and benchmarks, you'll be hard pressed to see more than 10-20 % (which is precisely why running a GPU on a x8 interface (or 2.0) is not an issue).
If it’s not true then you are actually removing the only major benefit of upgradability all together. Just get what’s available today since it’s fast enough and will be for some time.
I was saying that the Blackmagic eGPU in an upgradeable form factor would mean that you get a 200-300 $ (or something like that) GPU enclosure with some electronics in it (Thunderbolt hub, power supply etc.). Then you stick a current GPU into it. This would make sense because GPU replacement cycles are much shorter than the interconnect upgrade cycle. That's why I said that most applications are still fine today with the over ten year old PCIe 2.0.
So just to reiterate: PCIe bus speed is simply not a limiting factor for the overwhelming majority of GPU applications.
And you’re right—PCIe 3.0 cards out today have plenty of performance for today’s applications. That why you would get a PCIe 3.0 card today. But it doesn’t matter if it’s fixed because you aren’t going to upgrade it.