As I never tire of repeating - It’s not logically possible to advance the interests of one group without disadvantaging the interests of another. Immigrants do not live in a vacuum and cannot therefore do “no harm”. If one migrant cost the exchequer one penny more than he contributes he does harm, if one childs education is compromised because the teacher has to teach in 30 languages,harm is done, etc etc etc.
Levels of trust in mixed societies, and therefore social capital are reduced as compared to more uniform societies, a process that eventually leads to social breakdown and conflict. That’s quite apart from the pragmatic arguments about space, the infrastructure, the use of natural resources, carbon footprints, etc etc.

Instead of trying to find a million different ways of saying “unrestricted immigration is good for you so shut up and be gratefull for your vibrant neighbourhoods and your enriched culture”, the left need to show that it is better than restricted or no immigration.
It isn’t so they can’t.

If immigration pushes prices up by 10% - relative to wherever they would otherwise be - that is an extra 20,000 on an average house.
That costs a 90% mortgage payer on a 6% mortgage rate 1080 per year, 1350 of pre-tax income.
That’s a big cost.

Leaving aside the quality of life impact in London where parts of key infrastructure - roads, rail and tube - simply cannot expand.
Or the impact on education of 30 different languages needing to be acommodated.

The problem is for people in different socio-economic classes, the people competing for jobs with recent immigrants. And even if the net short term impact is zero, if that factors into benefits for the middle classes (cheaper loft conversions!) and costs for the disadvantaged, then that can hardly be seen as progressive.

“But absence of proof is not proof of absence.” - Ah, so you are are Rumsfeldian when it comes to epistemology too!