The issue is though that “segregation between state and religion” is a cultural trait. It’s not something that every culture values, nor is it something that inevitably happens.
In fact, it’s almost certainly a minority opinion on a global level. Particularly in (although not exclusive to) poorer non-western countries which tend to be much more conservative and religious.
A small number of conservative immigrants won’t hugely impact views in the host country, but a sizable number (particularly if they are concentrated in certain areas) absolutely can.
Why does it need to be equal to population?
I’m not saying that all immigration is bad, but rather that above a certain level it gets difficult to integrate people. For european countries this is a much lower number than the US, since populations are much lower. At the same time, there are many more refugees than in the US.
It’s a genuine challenge here in Vienna, for instance, at the moment because recent immigrants make up a large percentage of school kids, who often have few language skills, tend to be very religious, and have extremely conservative views on things like feminism and gay rights. Unfortunately, their views tend to self-reinforce rather than become milder over time due to being the majority view among their peers/in their school/community.
You can’t really blame the kids, obviously they are just a product of the culture they grew up in, however you also can’t just ignore the issue. There isn’t any mechanism for preventing immigrants groups from clustering in specific areas (and I don’t think most people would be in favor of anything that draconian)
In an ideal world, maybe there is a perfect solution, but the reality is that the current system is facing a huge challenge. Like it or not, this is directly tied to immigration rates.