Fort may be wealthy, but that doesn't mean individuals within that fort feel equally blessed with that wealth...
(But that probably needs Economics to be functionally reimplemented, rather than go on the basis of 18 socks have just been made, but 72 dwarves are currently sockless, who gets two, one or no socks, purely by chance,. Or who picks up two, then stores them as they replace them with a better two by a now slightly more competent socksmith, then does the same again shortly after, and all before anyone else gets a hand (or foot) in on the supply.)
Except the player simply notices the lack of socks in their stockpile and therefore orders the making of more socks and everyone gets socks, even if their distribution is still random.
The present situation is how any sensible player is going to run their fortress what you can 'Economics' is just basically going to be something to be hacked/overridden/exploited until we are back to the way things work at the moment. A competent human player is going to game the economy so that fortress wealth equals individual dwarf wealth far more effectively than the AI is going to do the same.
This however is beside the point; since that was not what I was actually talking about at all. What I was actually talking about is that moving from the old player fortress to the new player fortress is not going to be an act motivated by wealth, simply because the older player fortress is RICHER than the new player fortress, likely until the population is maxed out. The player fortress gets richer and richer over time, the only point at which poverty and scarcity are actually a thing in Fortress Mode is at the beginning; so greed will lead nobody to join a newer player fortress from an older one.
It is not wealth that drives colonisation, it is wealth that drives urbanisation which is the exact opposite of colonisation. What drives colonisation is the need for extractive jobs, basically a colony typically develops in the following fashion.
1. Extractors (such as farmers and miners) arrive in a area because there are not enough resources in the established area to provide them with jobs.
2. The resulting congregation of people create a demand for local service jobs.
3. Servicers (such a tavern keepers) arrive to meet the demand for such jobs.
4. Servicers create a demand for manufactured goods that are initially imported from the homeland.
5. Producers (such as craftsmen) arrive because they can meet the demand for manufactured goods outcompeting the homeland (this is the stage at which conflict between the colony and homeland occurs).
6. We have a fully built up urban centre.
Colonies do not offer a better quality of life than the homeland can offer. They offer particular jobs that the homeland does not provide a sufficiency of to meet the overall demand.