Just curious, among the critical stuff just 
    holding on, was there also a whole pile of 
    departments and teams doing work that could 
    cease tomorrow and the company would blink 
    and move on?

I get what you're saying. For any given team with a public-facing product you generally have perhaps 20% of the staff keeping things running and the other 80% of the staff is working on new features, reports, enhancements, customer support, whatever. You could eliminate them and while it would diminish the company in various ways, the other 20% could certainly keep the lights on.

However it's worth noting that's not what happened at Twitter; there were very specific and explicit reports that the "keep things running" teams were hit just as hard by layoffs/resignations as the other departments.

So there was real justifiable concern there.

There's also a lot of things that can go wrong during sloppy and abrupt handovers. Like... you fired the guy who manages the domain renewals. In the chaos of transition nobody picks this role up. One day 18 months later you realize "twitter.com" has expired. Or whatever. Even if the remaining staff is sufficient to keep things running, there are thousands of these little process interruptions.