Hoping for DDR 5 or PCI-E 5 support in Zen 2 is equivalent to hoping that Zen 2 is at least a 2020 architecture rather than a 2019 one. This is because:

1) PCI-E 5 isn't even standardized yet

2) Mass production of DDR5 memory chips might begin ramping up in 2019, but it's unlikely that DDR 5 memory sticks will be readily available on the market until 2020.

3) It takes a really long time to engineer a cpu architecture and bring it to market (generally a year or more for even just a revision of an existing architecture).

4) Adding support for a new memory or PCI-E version requires a new socket and a new chipset, which creates even more work that by definition can't be "done" until things are fully standardized.

AMD are going to support DDR5 and PCI-E 5 eventually... But they want to release a new architecture revision on their current socket and chipset now before it's reasonably possible to do that.

Edit: All this is to say nothing of AMD's commitment to potential partners to keep the underlying socket and chipset the same for a reasonable period of time, which is critical to planning supply chains and upgrade paths for major partnerships.