There are spaces in International Schools and local schools, just not the ones that people have their sights set on just like not everyone gets to go to Harvard or Oxford... That's the way it goes with PRIVATE schools...
Expats don't HAVE to choose ESF schools, they receive subsidies from the government and in return, they are to first serve the local community who pays their taxes. There are plenty of schools where foreign expats can register their kids regardless of where they live.
Knowing how some parents are, they would put their kids on every possible list, wait until the last minute and cause all kinds of aggravation in order to delay or make the acceptance...
By the way, under your logic... the expat would put the child on the wait list and time his/her arrival so that they have the flat by the time the acceptance arrives(or not). That really doesn't make much difference except perhaps being slightly higher a longer wait list. You would still have no more guarantee to have a place, only maybe a better chance but since the list would be longer, there could be more people ahead.
What you really want is be able to secure a place before you arrive in HK which isn't possible in any scenario and for good reasons.
This "logical" debate reminds me of a situation I had to deal with where I was labeled a knucklehead administrator. Bus route is predetermined with certain stops where kids are picked up. Parents comes in and ask if it's possible to have their child picked up in their building since the bus passes in front in the morning. Answer is of course no, they get upset and feel the school isn't flexible, it's no big deal, only a minute or two yadi, yadi, yada... Flip side is that traffic is heavy coming into the school, if one parent is accommodated then all parents making the request would also need to be then possibly creating much longer delays causing larger traffic problems because timing is important. Many, many, many parents come in and see situations with thinking only of their own situation. As I said, I would want the school employees to serve the existing paying community with their time and not a whole bunch of possible future attendees.