What are the incentives for improvements (or even maintaining the current standard) if they pool the services?
Certainly, during build out having the ability to plan your own network is a competitive advantage. Long-term centralised network ownership that is independent of the operators is the most likely outcome. Indeed this has already been happening for years as equipment get into the business of supplying the network rather than just the kit. You still need to balance the demands by the networks of lower costs with those of capacity and coverage. The model might be the rail network… the experience of which varies from country to country.