A server with nothing on it is like a field of weeds. It's just taking up space.
If a server with nothing on it is just taking up space, then the users will have no problem spinning their own up and replacing Reddit or whatever.
The community and the eyeballs are what is valuable, and Reddit holds them captive not due to any incremental value they provide, but due to network effects. Lots of people or companies would immediately replace Reddit if the quality of the server or UX or UI was what mattered -- but cannot because the audience is captive.
Killing the apps represents a unique "digg moment" of pissing off users enough to bother migrating.
A server with nothing on it is worse than taking up space, it's an investment of energy and CO2 release to make it and ship it around the world, and if it's powered on then it's taking electricity probably from fossil fuels and turning it into waste heat.
[1] https://en.wikipedia.org/wiki/Beneficial_weed
[2] https://gardenerspath.com/plants/herbs/edible-medicinal-weed...