Actually, their numbers have increased very rapidly, and they now are approaching 200,000.
The policy is not to give the farm to the oldest son, but to buy farms for all the sons. Generally, the father is still working and quite young (mid-40's) when his first son is ready to strike out on his own and start his own family. It is only many years later that most of them retire and pass the farm on in their family or to another Amishman.
Since the Amish do not participate in modern consumerist culture, don't own cars, don't own mechanized farm equipment, don't buy insurance, don't use electricity, don't pay social security and medicare taxes, and buy their assets generally unencumbered by debts their farms (and other businesses) are very profitable, and they have a large cash accrual each year that permits them to save towards these major purchases.
What you are perhaps getting at has been two tendencies: the extremely high price of land in Lancaster, PA, forcing many Amish to migrate to cheaper parts of the country such as upstate New York, western Pennsylvania, Kentucky, Wisconsin, etc. to found new colonies; and also a new tendency to open up small manufacturies or construction trade businesses instead of farming.
I'm not aware of any Amish living in cities. Some do live in small towns.
The defection rate from every authority I've read remains about 1 or 2 in 10. Most who do go leave to become conservative Mennonites, who are nearly indistinguishable from the Amish except for slightly fancier dress, and permission to own cars and tractors. Anything more is too extreme a culture shock in most cases.
I do not think the success, such as it is, of the Amish is very relevant to the eventual fate of Ave Maria. That is because the state (I use the word in its socio-political rather that uniquely American form-of-government sense) is not threatened by the Amish in the way it is threatened by Ave Maria. Or even if it is really threatened by the Amish as well, it does not perceive the threat. That is because we won't see America en masse abandoning electricity, modern dentistry, and such. But we may very well see America adopting local law consistent with the Divine Law, and if it does, these communities will prosper not only in the spiritual sense but also in the modern economic sense. And that would mean the end of the state's client classes, state skulz, state-protected pornographers, and state-protected abortionists. I think, all the guns will be out for this town faster than you can say "Ave Maria". Or "Waco".