Everyone knows what is going on and keeps the status quo, but a shock (often external) occurs to ignite the dry wood, and disruption erupts.
Demonstrations and riots can be healthy releases of tension in democracies. Bottle-in the regular microquakes, and you get devastating earthquakes instead.
I would say Russia, China, and North Korea (all well-known to disappear people) have been incredibly stable. Even countries that were quite liberal (e.g., Iran, Myanmar) have worn their populations down into compliance in their major cities. Even the liberal West is borrowing from the same playbook (e.g., police raids on investigative journalists, politically motivated prosecutions).
The strategy works, but as you imply, maybe there's a breaking point... or maybe totalitarian states have figured out how to get as close to that line as possible without crossing it.
One of the most effective ways to maintain an authoritarian regime is to proactively prevent a critical mass of revolt from forming. You just need to keep a steady low volume of arrests and disappearances going for people who express discontent. If not enough people are actively expressing discontent, you fill your quota with members of a group or area that you see as a risk. Move surviving prisoners and parolees every few years to prevent them from forging strong ties. History shows that your people will stay silent and meek, clinging to the belief that they personally have done nothing wrong.
A means of communication which authorities cannot snoop on is absolutely essential for any oppressed population that yearns to breathe free.