Not every form of genocide involves gas chambers and campaigns of mass murder. The UN's definition of genocide includes the mention of 'conditions that are designed to bring about the end of the existence of an ethnic group'.
Importing millions of non-whites into the country who have birth rates up to 5 times that of white Britons, while simultaneously forcing white communities and groups to accept these people through mandatory diversity quotas and shaming language, all the while blathering on about how white people don't actually exist or have any culture of our own whereas everyone else does, are all mentalities that if enacted together would lead to the ethnic displacement of white Britons in their homeland and their eventual genocide by out breeding. Once a critical threshold of non-whites was reached it would be electorally impossible for white Britons to ever be represented unless by sympathetic non-whites backed by their electoral majority.
Despite what leftists tell people, ideas such as diversity, minority rights and tolerance are largely only believed in by white Westerners, Hispanics and East Asians, with whites being the most ardent supporters and originators of such ideas. There is no current evidence, either in the prevalent mentalities that we find in non-white African/Muslim cultures, or in the politics they have in their home nations, that suggests that a large or even a sizeable percentage of them hold such concepts as minority rights in any regard at all. Thus people also raise the possibility that when white Briton become a minority here, we will be actively subjugated by the non-whites and eventually, possibly physically exterminated.
This isn't baseless paranoia, as groups like ISIS have attempted the genocide of minority groups like the Yazidis. Now you can say that not all Muslims are genocidalists but they don't all have to be for them to take power. The majority of Syrians aren't genocidal maniacs and yet much of their country is controlled by people who are, because enough Muslims are genocidal maniacs for those who are to form a serious army and seize power.
Even if a physical genocide wasn't attempted here, the world that we are making for ourselves is going to be a horror. Germany has taken in 800,000 refugees this year alone, most of them Muslims. That's 5 times the number of native German births, and if they take in more refugees next year and the year after, when we factor in the fertility rate of these people, they are inviting upon themselves the statistical certainty that their country will become majority Muslim. The left wants us to take in similar numbers here.
Many on the left are indifferent to this or believe that a Muslim majority Germany or Britain would just be exactly the same but with people with darker skin. Yet if we look at what these people largely believe, there is no evidence that our culture will remain the same and every single piece of available evidence points to the fact that it will make our country demonstrably worse. The crime rate of refugees from these countries is absurdly high, with most of the violent crime aimed at white people. They are in their majority against Western notions of gay rights and women's rights, and notions of 'tolerating the minority' are totally alien to them. The only reason such concepts are present in their countries at all is by Western export. They haven't arrived at the same cultural conclusions as white Westerners mostly have and there's nothing in the world right now that leads me to believe that being a minority white population under a majority non-white/Muslim population would be anything other than outright subjugation and Dhimmitude. This is why we call it genocide.
The statistics cannot be argued. If Germany carries on at their current rate they will be majority non-white with Islam as the state religion within three generations. And they're doing this because of a picture of a dead Syrians boy? This is insanity. If you believe that being a minority white population under a Muslim plurality and non-white majority would be just fine and things would stay the same, I'd strongly ask for some evidence that that would actually be the case and isn't just a Utopian fantasy.