Apple are expected to wait until 5G matures, then launch a product that takes full advantage of 5G’s capabilities; rather than rushing a product to market that has no inherent reason to exist, other than claiming the ‘first to market’ crown.
Apple does this with many technologies — and it works.
5G may be live in South Korea and starting to roll out internationally, but its main benefit (sub 1ms latency) will require substantial infrastructure upgrades and probably won’t be active until 2020.
Until then, we have gradually increasing download/upload speeds, which will enable interactive benefits like streaming 4K — not quite as ground-breaking as 5G’s promised low latency.
And then app developers have to create experiences that leverage lower latency (eg. remote AR — which Apple have expressed significant interest in).
So Apple waiting until 2020 to launch a 5G handset sounds very sensible.