Christchurch is still a war zone AFAIK. But I agree with your general point, which is that the venue has to move from time to time.
London is the biggest city in the centre of the largest population of rugby players and supporters on the planet. If it cannot sustain a Sevens tournament, we are all fooked. (Incidentally, the Middlesex Sevens used to be pretty big, back in the day).
Singapore is not Hong Kong, never was, and never will be.
As I have posted elsewhere, probably boring everybody to tears, Hong Kong was an overnight sensation - although it was a long night, 20 years. Plus, it has always been a far more vibrant city. No rules. Singapore, on the other hand, is still a nanny state. Different culture.
If Singapore has a potential expat audience for Sevens, surely the audience will turn out for the Tens?