It is anyone guess, so I will add my own for the fun of it.

From my experience, I believe that CX F demand comes mostly from wealthy individuals and awards (often upgrade from paid J).
Top executives still use F if they are in a booming industry; for a short trip to close a deal; if they are significant stakeholder in a profitable firm...
Many reasonably-rich people do not mind spending a few extra thousand USD, for that purpose, even for leisure travel.

Vaccine will just be an additional tool besides other measures like masks and social distancing. Many experts are worried that the 90% number is optimistic because immunity might not last long and the virus is mutating (like the common flu virus). My thought is that there will be increased demand from wealthy individuals to lessen the transmission risk. The motivational role of awards will still be there. Frequent paid J pax are very happy with a periodic upgrade to F.

There is no doubt that demand for travel, whether leisure or business, will be lesser in the coming years, once travel rebounds.. But for those planes flying, a premium airline like CX can hardly remove F. Airlines will be fighting to fill their large number of J seats. That will be the big challenge. Like QR , they might start unbundling their J offer. They will engage in more cost cutting. F still has a place, even if only as a premium-J product.

Just a guess. But that is an issue facing all airlines. And answers might be airline-specific. For example, BA with mostly 14 seats in F, is in a different position than AF with only 4 seats on a few planes. etc...