> What is so wrong with legal immigration rather than illegal?

Nothing, but you have to be sure when they say "we want to get rid of the illegal immigrants" they don't really mean "we want to make all immigration from shithole countries illegal and get rid of people from those places", because they like to play those word games. This is what happened in the US.

People married to immigrants, who love immigrants, who work with immigrants, and even immigrants themselves heard Trump say "We want to deport criminal immigrants" and voted for that.

Next thing you know they're deporting your wife. Then they start chasing away foreign students, and companies who want to work with and hire foreigners are put on shit lists. Then they start narrowing pathways to legal immigration to create more strain on the system. Then they remove protected status from legal immigrants creating a population overnight who are illegal. Then they start looking at modifying the Constitution to change what makes someone a citizen in the first place, and whether they can retroactively strip people of that right. Then they start looking at naturalized citizens to denaturalize them.

Eventually they start going after political opponents who are immigrants, raiding their homes and threatening to deport them and their families.

These are not hypotheticals, it's where the US went in fewer than 2 years since giving the "just no illegal immigration, what's wrong with that?" crowd free reign to do whatever they want. And we specifically got here by taking the reasonable idea that illegal immigration is undesirable and running with it until the anti-immigration problem is far worse than the illegal immigration problem.

So if you think 200k to house an immigrant is bad, if your country is anything like the US (it is) soon enough you're paying billions upon billions for an unaccountable domestic paramilitary force that deports people to foreign torture prisons and executes citizens in the streets. Then you have a real problem.