I also believe that there is an optimal level of immigration, which is relatively high, and not limited to the most skilled. Modern, first-world societies still have jobs which are relatively unpleasant and don't require much skill, for example in agriculture. Jobs that the natives don't want to do, but an immigrant would. So even the migration of unskilled labor can create those win-win situations. While it is extremely difficult to calculate what the optimal level of immigration is, it is at least theoretically obvious that this optimum must exist. At some level of emigration from country A to country B, country A will actually hurt from the loss of labor force, while country B won't be able to accommodate any more.

The European Union is currently in a migration crisis, mostly from about 700,000 people having migrated to Europe from Middle-East and African countries, often by boat over the Mediterranean Sea. The USA is likewise having a migration crisis, in their case mostly about 1.6 million people that came on foot from South or Central America over the Mexican border. In both cases, the real problem is not that migration exists, because if it didn't exist at all we would actually all be worse off. The real problem is the legal framework under which the migration happens, with a secondary problem being the level of migration, although for me it is hard to say how close or beyond the optimal it is.

To not be too politically correct here, there are a lot of shitty countries on this Earth. In most cases, a shitty country is both politically and economically shitty. For example, over 7 million people have emigrated from Venezuela over the past years, but it is impossible to say how many of them left for political reasons, and how many of them left for economic reasons. The United Nations Universal Declaration of Human Rights of 1948 and other subsequent treaties established that "Everyone has the right to seek and to enjoy in other countries asylum from persecution", and that is a good thing. But when there is a large number of people arriving at the southern borders of Europe or the USA, it is nearly impossible to determine which of them are actually persecuted in their home countries. I am certainly no friend of Maduro and his "Bolivarian Republic of Venezuela", but I do not think that they politically persecuted over 7 million people. According to Amnesty International, "between 240 and 310 people remained arbitrarily detained on political grounds" in Venezuela. That is bad, but still suggests that a large percentage of the people who fled Venezuela did it primarily for economic reasons. And why would those reasons be any less valid?

I think that the first world is shooting itself and others in the foot by basing its right to immigration on asylum from political persecution. Yes, political persecution is bad, but so is your children starving. And if you consider the kind of person who is actually most likely to get politically persecuted, they are probably a lot better educated and a lot better off than most economic migrants. If we base our right to immigration on compassionate grounds, why would we want to limit it to an elite fraction? What is the moral basis of saving the politically persecuted journalist, while sending back the starving agricultural worker and his family?

Processing of asylum claims in first-world countries are a mess, pretty much everywhere. The EU just agreed on a compromise that allows to lock up asylum seekers for longer. In most places the asylum seekers don't have basic human rights, like the right to work, or the right to free movement. And all that because the receiving countries are trying to make the impossible distinction between political and economic migrants. It is somewhat perverse that a person who wasn't politically persecuted in his home country but left for economic reasons ends up locked up in a camp in the country he migrated to, based on a right to freedom from political persecution. Much of the current crises in Europe and the US are basically those asylum prisons being full, which certainly isn't the right way to determine how much immigration we need. Asylum rights lead to very bad "solutions", like the US expulsing Venezuelans to Mexico, not Venezuela. It "saves" the US from getting it wrong, expulsing a Venezuelan to Venezuela, only to see that person get jailed or killed there for political reasons. But neither that nor a border wall actually solves the underlying problem, and causes a lot of problems in countries that have the bad luck of being geographically between the shitty countries people are fleeing from and rich countries people are fleeing to. Everybody can see that the current system isn't working, but nothing happens, even when right-wing parties are in government, who exploit the general dissatisfaction with the migration system for political gains.

At this point in time, rich countries setting generous numbers of allowed immigrations per year and deciding who to let in and who not on a lottery system would cause less harm than the current asylum-based system. At least the people who were let in could start working and be integrated immediately. Yes, expulsion of illegal immigrants is always going to be contentious and difficult, but unless you want to establish a right to global free movement, you will always have to deal with that problem. Regional rights of free movement work quite well in the European Union, and could be a model for other continents, giving actually politically persecuted people a place to go. Much of the current migration crises is due to a legal situation that is nebulous and impossible to correctly enforce. A much simpler system wouldn't be perfect, but still a lot better than what we have.