In Fallout Shelter, there will be no incentive for players to expand his/her vault after some time, as you will soon be able to deal with anything the game throws at you (namely radroach infestation) with what you have. Unless you find joy in just collecting more powerful items, you have no reason to continue playing.
That makes me wonder, will there be a reason for players to expand their settlement and keep the population happy? I understand that there are people who find joy in creating elaborate buildings and seeing their community thrive, but will there be incentives for more, idk, materialistic people?
I think if people in your settlement pay you some kind of taxes in the form of bottle caps or (sometimes rare) scavenged resources for crafting, that gives you a pretty good reason to expand your settlement. This may sound selfish, but if satisfied population provide more caps or better items, you will have an incentive to spend (at least part of) the tax to raise their happiness.
I hope similar systems will be actually implemented in community building of Fallout 4.
Edit: To be clear, I am not skeptical of the building sysytem. Along with new crafting system, settlement building is one of the features I am most looking forward to. I am just saying the more reward for players, the more appealing the new feature would be for them.