30 Hz is fine for playing movies, the overwhelming majority of which are 24 fps, which is realistically almost all the 4K people will be playing from a device with USB-C out to HDMI in (i.e. from a phone, tablet or laptop to a TV/monitor)
If you are gaming in 4K@60fps or whatever you will have a device with HDMI out and connects with a standard HDMI cable to your 4K display.
The long term fix will be yet another speed upgrade for USB to 20 Gbps, which will allow 4K@120 fps 4:2:0 at 10 bit color depth - that covers the highest possible spec for future 4K broadcast TV.
I wonder what they will call it, supercalifragilistic speed USB?