It'll all work out in the end
The cost of entry for mobile phones went down. 5G allows the cost of entry for the network to drop, too. If anyone can plop a 5G base station anywhere as long as they can connect to the internet somehow, then MNOs might be defined by their customer service.
We have internet (IETF) equivalents for most aspects of a mobile network offer, such as rating. Apple broke the anti WiFi cartel with iPhone because networks knew people preferred it to the cludgy smart phones with limited apps pre-iPhone.
Once people discover they could go to any service provider and be directly billed, they will go to the MNO that gives them the best aggregate billing. It's a bit like what happened in the USA with deregulation and the baby Bells. Customers found they were being billed completely different tariffs and sometimes different bills.