It's still a city in England, it's just more culturally diverse than ever. Why is that a bad thing?
It's a question of identity.
Why are such efforts made to retain the appearances of listed buildings? Why when you mention giving up the pound in place of the euro do people get so emotive about it? (Financial reasons notwithstanding). Because they value the historical significance of it.
London is the capital of the UK, it should bear a greater resemblance to the rest of the country.