It would leave a fairly big gap in the lineup with nothing to compete against Intel's X299 platform. AM4 is lacking in memory channels and PCIe lanes. Epyc has much lower clockspeeds, much more expensive CPUs, and more expensive motherboards than Threadripper.
Well, that is what first-gen Threadripper was. Same socket and all, but with half the connected DDR lanes and a pin telling the motherboard it's not EPYC.
I know it's not a big difference, but given the changes to IO and the 16 core consumer version, I don't see why there would be any internal difference to EPYC this time around (which this article claims will have a variable number of chiplets).
As Lisa said, TRs were distinct to Epycs; I guess using UDIMMs vs RDIMMs and much higher base clock (except for the high freq EPYC 7371) led to a few changes.