This is a silly argument: because you are unable to quote the name of a potential successor to a person, then that guy must be irreplaceable. I am quite sure out of the 15 politically appointed ministers, a majority of the Hong Kong people wouldn't even have heard of many of their names before they got appointed. Apart from a few lawyers, I am quite sure none of you have heard of Wong Yan Lung before he was appointed as the Secretary of Justice. No one would have possibly quoted his name when asked who can be the next Secretary of Justice, but he turns out to be one of the most popular ministers, out of this highly unpopular cabinet. Could anyone think of a successor to Ambrose Lee, the Secretary of Security? Hardly. Will our public order collapse if he has to leave his post somehow? Not at all.
I do not understand how on one hand, we claim our city as a "financial center" while on the other hand, we think it is impossible to find just ONE person, out of 7 million people, to maintain that automatic currency board system. If the status of Hong Kong as a financial center depends solely on the existence of ONE person, that is truly dangerous and we should try to make it less so. After all, it is not uncommon for public officials to be unable to carry out their duties, for one reason or another, so how can you say someone cannot be replaced?
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