In my opinion migrants cause desaturation of job markets in their country, wage depression in yours, wage inflation in theirs, large supply chains (due to engaging in a practice that encourages specialisation of logistics and production on a global scale), etc.
We have seen the negative effects of such for quite a while now, the majority of the reason why migrants are seen as a positive is because it allows labour to become artificially competitive and restrictive, it allows rather poor business practices. Even in terms of voters, you can use immigration to your advantage by being the party they stand to gain from. You introduce a whole new interest group, that is almost entirely dependent on corruption, etc.
Then you can think about how a lot of immigrants do not even actually want to be in your country, so they are taking a job, and chucking away income from said country back to their origin
You have simultaneously incentivised developing nations not to educate their own workforce, to develop their own educational institutions, as well as deprived them of the talent and manpower to do so, whilst making all services and supplies more costly in their nation, on top of making it so the entry level of jobs in your own country is squallid, that almost everyone will get paid rubles for a job because now it is saturated with people, and worse yet, it is saturated with people who would have accepted a few dollars an hour or less, let alone anything close to minimum wage
And then you have an experiential loss of the field, as no one can pick up the trade and compete, no one is teaching apprentices, businesses collapse, they get bought up by large international chains who can afford to employ from overseas and across a country and
Immigration is fantastic for quarterly margins, especially temporary, it is an awesome way to massively increase profits, especially in larger entities, that is why almost no political party (even the ones that actually say they dislike it and want to stop it), never touch foreign visas if they are the boldest of the bold, but like most, do not even actually change anything, a.k.a. the Coalition Party of Australia, even the Labour party has railed against wage depression. Guess how much stricter, how many visas were removed, the standards etc,/., changes to award wages,? None. The same is true of Republicans, Democrats, you can go into Europe as well, same story. British government does it as well. It is very rare that they ever go against the grain.
Then you can also think about the housing issues, you know how we have an enormous amount of difficulty purchasing land these days? Have you considered that an artificially rising population that comes here with jobs irrelevant to the field and so on creates a disparity in the services available, and that it becomes impossible to construct houses at such an artificial rate? No, this is not the only variable, things like foreign ownership are among the largest factors in it, but you still get the point.
This is on top of the fact that geopolitics and ethnocultural divides alone create issues and instability (for the most part), and so on
If you ever think something is just a straight win in life, you probably have not reasoned its benefits and detriments enough, immigration, depending on how it is managed, can be genuinely beneficial, with much lesser impacts on the quality of life in a given nation, but it is exceedingly rare that it is, and it also very prone to error to try, and there is almost no incentive for immmigration once you change a lot of the laws that allow people to abuse your nation in such ways
Almost all of the immigration in the world is going to go to the other 50 countries that decide to enable these kinds of things, easily and readily
You also have to think about the fact that not having lots of immigration necessarily makes you uncompetitive with the rest of the world for the most part, a whole bunch of stuff really, and you would have to actually face the impacts of your fields being lost, and your population dwindling, etc.
Even the point about degrees, part of the reason job standards are becoming so irrationally high? You guessed it, immigration. It is really hard to compete in any part of the workforce when your country can hire someone for less who already has ten years of experience and degrees.
Competition can be healthy, it does not mean that it inherently is.
