@Reformed J I think it’s stupid as well. But with regards to the working class, it makes sense because immigrant are indeed willing to work for less. That is their point.
Their argument which is line with your supply and demand point:
1. Potential negative impact on real wages. It is argued that low-skilled immigrants put downward pressure on wages. The argument is that an increase in the supply of unskilled labour enables firms to fill vacancies with lower wages than previously.
(This is the argument used against the Polish and Eastern Europeans. This argument is the one that inspired the working class to vote Brexit by parties such as UKIP)
2.
Pressure on public services. Immigration and a rise in local populations put higher pressure on social services, such as schools, hospitals, roads and public transport.
3.
Real GDP per capita could fall. Often supporters of immigration point to how it increases real GDP, and this is true. A rise in the population will ceteris paribus, increase national output. But a more useful measure is GDP per head. If immigration is of low-skilled and/or those not in labour markets, it will lead to a fall in real GDP per head.
4. Structural unemployment. Immigration could lead to some displacement of native-born workers who then experience structural unemployment. For example, if migrants gain unskilled labour because they are willing to work for lower wages. Those native-born low-skilled workers may find it harder to gain new employment in higher-skilled occupations.