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.
Which excludes the vast majority of functionality of Google/Facebook, and most other major web properties.
Whoever is pushing for this is an idiot then. Verisign, for example, has had 100% DNS uptime for the .net, .org, and .com root servers for ~15 years because of their mixed environments. In every one of their POPs they tend to have at least two racks of equipment with:
This is all pretty much randomized at each location. As a result, a bug in one piece of the stack (hardware, software, driver, security, etc) will not take down their service completely.This is how you run a reliable global-scale service. Anyone who plays the "it's just easier if we all use ____" is in for a big surprise when their entire infrastructure is at risk due to one bug.