A starting point in any argument about human affairs is the principle of freedom, that people should be able to do what they want, including moving anywhere they want, and buy and sell anything they want, unless there are good reasons to impose restrictions. There are people around who do believe that people should be able to move to and from countries at will, and there are also those people who believe that we should all be able to trade goods and services across national borders without any tariffs or other impositions. There is the strong economic argument that giving people freedom to move labour, goods and services leads to the most prosperous economy, simply because the goods will be produced in the best place for their production, and people will move to the best places for the application of their skills. For example, I don't think anyone is suggesting that the states of USA should impose tariffs and immigration checks on their borders, basically all people in USA benefit from the fact that they can move themselves and their goods to whereever they want within the country.Why is diversity an end in itself, such that Israel having "much more diversity" is a desirable outcome? It is one thing to say, "This potential immigrant does not share our broad societal identity, but they are willing to comply with our norms, not challenge our prevailing culture, and work to enrich and improve our country," but quite another to say, "We actively want people who differ from our prevailing norms." What does Israel stand to gain from the presence of Hindus, for example? What can Hindus provide for their society that Israeli Jews cannot, such that one ought to actively desire a meaningful Hindu presence?
I want to stress that I'm not suggesting people whose identities differ from the broad norm cannot meaningfully contribute to a society. Rather, it's unclear to me why they should be actively desired in most cases. After all, even if one argues there is enough space in Israel today to accommodate immigration, today is not tomorrow, and Israeli Jews have fertility rates which exceed replenishment levels. Any space one gives away today to foreigners is space which cannot be filled by one's own descendants tomorrow, and the same is doubly true for political space, since the children of those immigrants will be voting, and will feel no less entitled to shape their home society than would anyone else.
I still do not understand why westerners seem to believe diversity is an unmitigated "good." It seems that sometimes it can be beneficial, and sometimes it can be toxic, so surely one must evaluate both on an individual and systematic basis?
Another argument in favour of such free movement is that mating between peoples of differing genotypes generally increases the health of the population. Ideas of "purity", taken to an extreme by certain historical rulers who married their own siblings, acted to weaken their health by restricting the gene pool. This is observed among Ashkenazi Jews where there are certain genetic diseases which are more prevalent than in the general population.
In the real-world (as opposed to the ideal world) there are nations which impose barriers to movement of peoples, first and foremost to prevent people with bad intent being allowed to enter. The fearis, in a country like Britain, that there is insufficient control of people entering, about half a million a year , many of whom cannot be traced. I think that immigration is to the economic advantage of people in general, but in England the population is now over 1000 a square mile and, to keep our "green and pleasant land", we cannot have a the equivalent of another provincial city added every year.