4G, just like 3G before it will get slower over time. The spectrum is still shared, and the more devices are connected on the 4G bands, the less bandwith there is for each device.
IMHO, 5G is not so much about speed as it is about latency and better handling of devices in high density areas.
Regarding the speed aspect, keep in mind that most smartphones (which are the primary 4G connected devices) are not equipped with modems that can reach the speed limits of 4G. They certainly could, but that would kill battery life. I expect something similar with 5G (although one the promises of 5G is lower power consumption). VoIP over 3G is often terrible. It's better on 4G, but, in my experience, it's often hit and miss. Having improved latency and speed means VoIP (and video calls) are, theoretically, less of a gamble.
(My experience with 3G/4G is a single datapoint and should not be taken too seriously, evidently)
However, cheap 4G is a good option for areas that have decent cell coverage but poor broadband (I refer to the usual copper lines for DSL). 5G can make the situation better, if only by making 4G cheaper. Deploying 5G means that an operator may have to upgrade their backbone ... which is good for 4G users too.