From what I'm reading in other comments looks like 5G primarily fixes issues that 4G has, so perhaps it should be called 4.1G.

In that case it feels like maybe it will just replace 4G.

It does fix the max number of devices and bandwidth, but it introduces drawbacks too: a lot more infrastructure needed, shorter range (worse signal for your mobile), worse mobile battery life. In my opinion 5G is not the upgrade from 4G, it's a different technology that will co-exist with 4G and be used in very specific cases.