Other huge companies have failed and left their flagship corporate headquarters behind.
A mixed use center like GM's world headquarters with multiple companies, hotels, etc could continue on should GM leave it. Which GM reportedly was considering.
It still would have been a huge body blow to Detroit but at least there are other tenants there.
A large cluster of office buildings like what exists in the HP park can be viably leased to a bunch of companies. If the total occupancy is only 25% these could be in the newest buildings while the less economically viable ones are demo'd and replace with something else.
With a large single building half the size of the pentagon (pentagon holds 23K employees)?
It's a valid concern for a Post-Jobs Apple. Apple went from sky's the limit in 1984 with Jobs showing the world the first Macintosh to free-falling in 1995 under Spindler.
Here's a reminder of corporate failures from that time period and I'd say the economic situation is worse than mid-90s.
You hope for the best but should plan for the worst. In this building you likely have to gauge the probability that a new single tenant would appear to occupy the building in the kind of environment which sees Apple leaving Cupertino for whatever reason.