Notspots are a problem.
As a very simple example, I used to be on a network whose quality in my house degraded over the last 5 years to the point where voice calls were frequently dropped or broken. I switched network a year ago, and my wide switched last month. This solves that one problem.
However, if a customer of that network comes to my house, they have no usable service. Similarly, there are areas I have been with my new network where I get no coverage. And all of this is in a densely populated area. It could possibly be solved by national roaming, at least in part.
When it comes to rural settings, things get even worse, to the point that there are many areas where you would get no signal on any network. This would be unaffected by national roaming.
I would suggest that the best way to solve this problem would be a separate network provider to cover these not-spots, as a nationalised infrastructure company, or a company which all 4 networks were required to pay a subscription to. Their purpose is to build out at least a 2G network into those areas which would be so unprofitable that the existing providers would not do it. The costs are shared between all the networks, and the service is provided to all of them.