Re: " How much better do mobile networks really have to be by 2030-ish?"
It may not matter where you are but in densely populated areas around the world 5G is already mitigating congestion mostly using sub-6 bands, much more rarely mmWave. In some cities where 5G "seems no better" what this really means is that 4G alone by now would have been much slower due to congestion. In other places maybe only one channel on towers run 5G while 2-4 others remain on 4G for the time being (each tower usually have 3-4 frequencies/bands), so 4G simply have better coverage even in 5G areas - something that will change over time.
And network utilization is only increasing so we need 5G to mitigate that congestion that's increasing in many places.
In Canada one of the channels being rolled out for 5G is 600MHz. It doesn't have a lot of bandwidth but very good range and penetration of buildings. So there will be better coverage, which is important here. 3.5GHz and higher is on the way for bandwidth and I believe Telus is testing mmWave which will only be relevant in very dense areas.
So, think of 5G as a more solid network, with more ability to handle load demands, better spectrum management and provisioning, and in some cases more bandwidth for the individual.