> Don’t we care about output per capita?

Not "yes and no", the answer is simply yes. You cannot simply flood your country with unrestricted migration from lower GDP per capita countries and not expect overall growth to slow down.

> Yes, output per capita is the primary measure of individual welfare but...

> our ability to service debt and social security obligations depends on total output.

Our ability to service social obligations and debt entirely depends on GDP per capita. Whilst they are both paid on a GDP basis, they a generated as a multiplier of capita. If you have 1 million people, and add another million people (of the same distribution), social obligations are also doubled, as will debt, but both delayed. It's not that complicated.

> We live in a welfare state, and this is unlikely to change anytime soon.

It's about to change now, the time is up. Governments world wide are now struggling to issue bonds at reasonable rates, there are no known mechanisms to unwind. The likes of Japan, a large buyer of the foreign bond market, starting to bring down its bond purchases, indicates this.

> Most immigrants worsen the fiscal position of the government.

This is especially true whilst you have a system already setup making a loss, such as the UK's pension system.

> Each immigrant into a rich country makes the position of poor countries harder.

Every doctor, nurse, engineer, etc, that we import is one less for their original country. What do we think that does to the original country on scale? What do we think that does to their growth?

> Affordable housing:

Many animals will not breed, and some even miscarry, if they are not in a suitable environment. Giving birth and raising children makes the mother/family very vulnerable. It seems that for all of our sophistication, the human race is no different. What we're measuring world wide appears to be an enormous economic deficit.