It is often done in a more pernicious way, of course. Few people are willing to admit that they are racist, first of all because many sincerely do not think they are (and indeed are not); at the same time, the majority of people absolutely refuse to live near the Third World whose merits (which they do not experience in daily life) they can otherwise extol in society. It is not incompatible. Another reason for hidden racism is the legal condemnation of "hate speech" in a number of countries, obviously.
Beyond the population substitution itself, which alone is a problem, when you see phenomena such as Somali immigrants wanting their children to grow up at home rather than in London, you can safely conclude that there is a problem. When you see Pakistani gangs kidnapping, torturing, raping and killing girls (and even boys) by the hundreds in multiple cities across the country, it's reasonable to question the benefits of their presence. Especially since to act on such a scale and with such impunity for so long, in addition to being covered by some unscrupulous police officers fearing the "racist" opprobrium, as it has been seen, they had the consent of the rest of their community certainly well informed of their depravity.
These are only some of the problems, and it seems to me that they are already well known, but while reading you I thought it would be useful to recall them.
The fundamental idea behind the refusal of substitution, a fortiori when it drastically drags down the country, is still to be able to appreciate the particularities of each place, and not to be subjected to a soulless global melting pot. To feel at home, too. Eastern European cities, for example, are much nicer and safer - no "part and parcel" there. Very pleasant.