Well Naomi we can both be saddened by each other then. I'm struggling to understand what is so sad about wanting someone to be allowed to do the job they were elected democratically in a free and fair election to do. It's disgusting for the man in the video to suggest that people should be barred from office for -- for what? Being Muslim? Bars on religions from holding public office is an attitude that ought to have died once we got over the whole Catholic/ Protestant thing in this country. It's sickening to see it rear its ugly head. Oh, and the "sharing platforms" thing -- if people can be tainted so easily by association, we could invert it and point out that Zac Goldsmith shared a platform with "known terrorist sympathiser" Sadiq Khan... does that say anything about Goldsmith? No, not really. Sharing a platform and sharing a view are very different things. How easy we find it to taint people by association, if we have some vested interest in doing so. (Should, perhaps, Khan have "no-platformed" the people you're complaining about?)
Apart from the obvious reasons why Muslim population growth can't continue indefinitely at the present rate, there's also the point that after all quite a few Muslims are being "westernised", at least to some extent. Among other things, this will mean that we can expect Muslims to have a more typically Western, low, birth rate; the immigration contribution to growth, too, is unsustainable (not to mention potential rule changes in the coming years that would cap the numbers anyway), for essentially the same reasons.
With respect to the apathy thing -- I don't entirely mean it seriously, but nevertheless around 50% of those eligible to vote in London's election did not. If, in the end, all the warnings about Khan are correct, then it's to those 50% you should probably turn first. In general, we get the politicians we deserve because of this lack of participation in elections -- it's a wider problem that hardly allowed "Just" Sadiq Khan in. Oh and, incidentally, you are repeating an assertion, also unfounded in any actual evidence other than supposition, that Khan effectively wasn't supported by the non-ethnic-minority Londoners. Who knows? Perhaps he could have won there too? Or do you have inside information on exactly who voted for him, and who did not? Your intuition may well be correct, but it is just intuition and not fact at this point.
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Your last paragraph is typical of the argument presented in the video above, and by others, wrapped up at least in a less aggressive voice, but it makes the same flaw. At the moment, "our country" includes Sadiq Khan, so far as I see it. There is nothing to the contrary that is anything other than a vicious attempt to smear someone because of his religion. As a result, Khan's election does not make me feel that I am faced with the total obliteration of this country.
On the other hand, that Khan's election has exposed the deep-seated prejudices people like you seem determined to pass on to the rest of us, dividing people as far as possible, *does* make me fear for this country. I had hoped we had, by and large, moved past such divisions. Apparently not. We risk tearing ourselves apart in yet another religious war.
What is my attitude in the end? It remains the same -- a point you have consistently failed to address (beyond the vague "links"). What has Khan actually done that suggests he is such a threat? You linked to an article yesterday asking if he would stand up for gay people, same sex marriage, women's rights, etc? Well, he has done. As and when that changes, then we can start to call for him to be ousted. Until then, keep an eye on him, by all means. But don't condemn a man for something you only think he might do. Judge him by what he actually does. Why is that such a frightening attitude?