I just looked at the spec sheet for the Gemalto PLS8 cellular modem, and the USB 2.0 connection is plenty fast for this modem. It is only a LTE CAT 3 modem with 100 MBit/s download and 50 MBit/s upload, so its USB 2.0 (480 MBit/s) is plenty fast.
Given that the max of G4 is 100 MBit/s downloads and the best average in places like Singapore, South Korea, Netherlands and Norway is around 50 MBit/s, the use of USB 2.0 isn’t important.
One annoying thing is that you have choose between the PLS8-US and PLS8-E, so people traveling between different parts of the world will have to buy two different modems. The late May update says that Purism is evaluating other modems, and I really hope that they find a modem that has worldwide coverage. Also the PLS8 only supports GPS and GLONASS, so the Europeans, Chinese, Japanese and Indians will be sad that their GNSS isn’t supported. More importantly A-GPS and LTO aren’t supported, so will be less accurate. I wonder if there are security implications to using A-GPS.
One thing that surprises me is the size of the modem chip. Look at the package sizes of the cellular modem and SoC in the Librem 5 compared to a high end Snapdragon that has both functions in it:
PLS8: 29×33 = 957 mm2
i.MX 8M Quad: 17x17 = 289 mm2
Snapdragon 845: 12.4x12.4 = 153.8 mm2