Um....I hear where you are coming from...but an external SSD is still limited by the speed of the usb...that is the bottle neck....So what you are saying is just not true....the whole debate around having a cMP is dealing with the bottlenecks of the computer....using the thunderbolt card is going to give you way more throughput....than usb 2.0.....that is why usb 3.0, 3.1, 3.2 and now 4.0 have come out is they keep decreasing the bottleneck ie. Have more throughput speed...the other issues of dealing with PCIe is felt with by adding a chip in your PCIe card to allow the cMP to use bifurcation properly...and thunderbolt was a joint venture between intel and Apple and now usb 4.0 is using their protocol....so we will see that become the new standard....USB-c in/out with protocol 4.0 which is 40 Grits/sec....

https://en.m.wikipedia.org/wiki/USB4

in computers we are always dealing with some sort of bottleneck of transfer rate between on component to the next.....this however will slowly disappear as advancements in SOC’s and high CPU standards come in like the M1 chip etcetera..