Sweden is a good example of what happens in scenario b). The effects are immediate:
- shortage of housing
- ballooning social security budgets
- extreme long waiting times to get health care service
- a marked increase in crime, especially in certain categories (sex crimes, robbery, arson)
and long-lasting:
- taxes need to increase across the board with ~2% (in a country already burdened with close to the highest taxes in the world)
- costs for state pensions stand to double in the coming 20-30 years
- the real estate market will need to adjust to the new situation (oversupply in the high priced sector, a shortage in the low and medium price sectors)
- the labour market does not fit the education level of the new population. Available jobs require higher education levels than most immigrants have or can reach leading to high unemployment among immigrants. The extensive social security system and the related high taxation level takes away incentives - and in some cases actually presents negative incentives [1] - to go from state benefits to paid employment.
The situation in Sweden is made worse by the fact that the country officially does not mandate nor stimulate assimilation of immigrants into Swedish society while the native Swedish (and assimilated migrant) population do expect immigrants to assimilate to a large extent, leading to increased segregation [2].
[1] It is fully possible in Sweden to end up with a lower spendable income when moving from state benefits to paid employment, mostly due to the fact that those on state benefits often get extra benefits for specific purposes (e.g. housing costs).
[1] Sweden does not have a single monolithic constitution, instead relying on four fundamental laws (the Instrument of Government (Swedish: Regeringsformen), the Freedom of the Press Act (Swedish: Tryckfrihetsförordningen), the Fundamental Law on Freedom of Expression (Swedish: Yttrandefrihetsgrundlagen) and the Act of Succession (Swedish: Successionsordningen)). An 1974 amendment to the Instrument of Government act states that immigrants can choose to keep and strengthen their ethnic and cultural identity instead of assimilating into Swedish culture.