I've got a strong sense that the vandalism here didn't, in fact, cause the plaintiff to lose any tenants. But a reasonable jury could perhaps conclude otherwise. As the Court of Appeal holds.
More importantly, I completely agree that an insurance policy that insures against "lost rents" does not pay out only if there's an existing tenant. If an event means that the place can't be rented in the future, that's an element of damage. If an insurance company doesn't want to cover that, they can say so.
And, perhaps, in the future, will.
Only causing people to hate insurance companies even more than they do already.