Your question is my suffering... I don't know why they didn't done same with X99 E WS. For that no excuse.
ASUS should know that USB 3.1 Port Card is useful for people with i7-5820K.
USB 3.1 is not so important to drop speed of graphic card on x8 and to leave people without options for Sound Card.
But it's still very important thing if someone choose motherboard for 3 years.
First my personal opinion is that every high end motherboard should have one
PCIE x1 for Sound Card and one PCIEx4 slot for something as USB 3.1or Intel 750 SSD, etc...
and than such small device can't make problem to graphic cards.
They should offer R5E with native USB 3.1 before 3 months.
USB 3.1 card everyone could install, everyone on any board.
And even to choose ASRock USB 3.1 Port card with Type A + Type C connectors together.
I think even old board could become USB 3.1 on that way.