As long as a sufficient fraction of servers at a sufficient fraction of Verisigns clusters has an uncorrupted set of data and is able to serve responses, Verisigns TLD zones remain up.

Pretty much the only thing that can go wrong in that case, assuming you have safeguarded the integrity of the zone is bugs in components outside their direct control.

It makes 100% sense for them to focus their efforts on ensuring diversity, because the class of problems that can solve for them makes up an unusually high percentage of the possible failure classes, and the nature of the service also means that most of the potential problems that this can cause is only likely to take out some proportion of their capacity that still leaves them with a functional system, so the potential benefit is higher for them than for most with a heterogeneous, and the potential risks are lower for them than for most.

For Google and Facebook, the systems are so much more complex that the tradeoffs are vastly different.