I'd hope that AMD will have mostly decoupled CPU performance from memory frequency by the time DDR5 becomes mainstream, preferably as soon as Ryzen 3 next year to improve its competitive situation against Intel's CPUs which are far less sensitive to memory clock. A $30 cheaper Ryzen CPU does you little good if you need to spend $40 more on faster RAM to match Intel's performance.
AMD use ddr4 at least untill 2020, so 2021 we may see Ryzen with ddr5 memory support. So it may be Ryzen4 by then.
But yeah! New memory standard is good. Older memory will/may get cheaper for short period of time!