But contrary to 3G, saturation of actual, human related client devices should be reached today. I agree that some mangling of frequency bands might be nice to accomodate for high-device densities (metro tunnels during rushhour are bad around here).
What I personally don't get about the whole 5G-craziness is why we (as a society made up of humans in contrast to consumers which fill the pockets of shareholders) need it? Today, we face a number of challenges which all result from our lifestyle requiring an unsustainable amount of ressources/releasing a massive amount of substances (be it fungicides (candida auris up front here recently), antibiotics or CO2).
Why do we "have to" go for artifical, industry-driven growth again, when we really should focus on increasing the efficiency (or even forgo implementation of ressource-using technology)? And 5G will entail a massive increase in power-consumption (more bandwith), the number of cell-"towers" [most frequencies applicable to WANs are already utilized – a lot of the promise of increased network performance now lays on microcells (e.g. every bus stop get's a basestation)] [1] and requirements on the network-clients (why stop at low-latency, if we can sell you 4K-video calls for your 8K-display...).
As for the "health effects": I believe that for classic networks the effects are negligible (though it's hard to reliably do studies on this. It's like with Dieselgate and the health issues...). What happens on the long term if you move around a city where every surveillance camera has a wideband (think radar and microwaves), 5-10W transmitter, beamforming that power to the receiving device unaware of any obstructions such as various bodyparts, which are definitely affected. I guess this is really something you should study (even industry agrees [2]). BEFORE DEPLOYMENT. Additionally the whole beamforming stuff with microwaves opens up really interesting possibilities: how about hacking your surveillance cameras+the basestation and then frying the eyes of some commuters sitting idly around. And with most of the devices operating completely on software and increasingly being single sourced, such stunts are becoming relatively easy to pull of.
[1] https://www.lightreading.com/mobile/5g/power-consumption-5g-... - "sponsored by Cisco Juniper Qualcomm"
[2] https://www.infineon.com/dgdl/Infineon-Health%20Effects%20of...