The seats are indeed designed to recline on some aircraft.
But the doors are designed to open too, overhead lockers designed to open, lights work, belts unfasten.
The key is at the appropriate time.
Long haul flights in darkness, most people want to sleep, blinds are closed, lights out, most people want to do the same thing and are encouraged to do so. Reclining after food has been served makes perfect sense. The domino principle naturally applies.
Daylight flights, especially shorter in length, it becomes a balance as there are different preferences. At the very least a recliner should have the conversation with folk behind.
Dickweeds who recline on a daylight shorthaul flight (if possible) and don't discuss deserve all the retribution they get.