Longtermism logically implies "Effective Border Security"

I believe that if you support the premises of longtermism, which I believe themselves logically follow from the premises of effective altruism in general, you reach a surprising conclusion that an important (not the most important, but important nonetheless) cause area is fortifying the borders of the first world.

My logic runs as follows:

1. The assumptions of effective altruism are correct - Doing good is good, therefore doing more good is better, therefore you should seek to do the most good you can with the resources you have.

2. The assumptions of EA imply longtermism - As opposed to the 8 billion human minds there are now, there could be quintillions of minds in the future experiencing unimaginable wellbeing, so making sure things go well for the future is the most good you can do with your resources.

3. Longtermism implies trying to ensure the survival of advanced civilization - To ensure that a future unimaginably better than the present comes about, technological civilization has to continue existing and advancing.

4. Mass third world immigration threatens advanced civilization - This is the most controversial point, but I think it is intuitive. Influxes of third worlders into Europe have caused a large amount of political and social unrest that has come to a head in the past decade and a half. Statistics from Denmark show that pretty much all third world migrants are a net negative on how much they contribute versus how much is spent on them by the state, while first worlders are a net positive. Authoritarianism has seen a serious boost because people genuinely prefer, by revealed preferences, to have a strongman in power if he can promise to reduce third world immigration. Horrific acts committed by third world immigrants like the Pakistani grooming gangs in the UK undermine fundamental trust in the social contract that serves as the bedrock of civilization. Further,  there are also robust IQ statistics showing third worlders are much less intellectually able than first worlders, and low IQ strongly predicts many negative outcomes. And quite frankly, it should just seem obvious that hordes of desperately poor people pouring into a civilization will destabilize it, as mass migration has destabilized and destroyed empires throughout history.

5. Things that threaten civilization should be opposed - Natural consequence of wanting civilization to survive.

6. Therefore, Effective Altruists who accept the assumptions of longtermism should oppose mass third world immigration - QED

I call this outlook "Effective Border Security" for lack of a better name. I think it suggests many interesting and counterintuitive things about which causes should be prioritized. For example, one of the most popular EA cause areas is saving third world lives by interventions like anti-malaria bed netting. This outlook would suggest such giving might actually be detrimental given total utilitarian and longtermist assumptions, because it produces people whose migration could reduce the odds of fantastically good futures for countless more people coming about, while only saving a comparatively small number of lives in the present. Instead of saving the third world, this would suggest we should instead be defending against them for the sake of the future. However, not everything implied by this outlook is stereotypically "right wing." I think one of the most important cause areas is combating global warming, because future global warming could produce climate-induced migration to the first world that dwarfs the mass migration from e.g. the Syrian civil war. While I think that third world migration is not the largest threat to civilization (that title would probably go to unaligned AI), it is a threat, which is important to recognize and act against. Therefore, if we want a glorious far future, we will want civilization to survive, and therefore we should act to reduce and guard against mass third world migration.