The first test for Hong Kong is after June when the school year ends. There is an inevitable exit.
Anecdotal evidence (movers booked in June-August. Second-hand furniture, appliance & club membership up for sale seems larger than previous years.

Crack comes if long term locals start moving their kids & primary residence abroad.
It a fallacy to think mainland population can replace a small number easily.
China working population is in decline and has been losing 300,000 every year (presumably educated middle class)

Same as Hong Kong demographics.
People move from China to Hong Kong because of regulatory & legal arbitrage and education.
lastly, look at South Africa after it lost 5% of the population (mostly mobile middle class)

Another anecdote of people planning to move primary residence is a car park for rent/ sales - this seems to be increasing.
you need a car if kids go to boarding school or you move the primary residence out of HK