The problem with importing millions of low earners into any city is the balance between tax revenue and services are irrecoverably damaged. London is one of the worse places for this.
https://www.google.com/search?ei=QP0.... _NL9HfuG6zs
The above link will show you how in the last few decades the poor n London have grown beyond the governments ability to support them. The type of support they need are expensive.
Housing = £20k to £50k per person.
Education = £73k per child
Healthcare = £3K per person per year.
So how do people on less than £30k a year contribute enough to fund decent public services?