My proposal:

I'm well aware that redesigning Castrum/Praetorium is not possible, and that the devs have learned from their mistakes and will not repeat them. However, these two dungeons are still required for every player. I propose a simple solution.

Before sections of long cutscenes in the game, there is a popup warning stating that multiple cutscenes will play, and please make sure you have enough time to view them. I propose another popup, upon unlocking access to these two dungeons. Something along the lines of "These dungeons are a cinematic experience, best enjoyed with people willing to take the time to enjoy them. We recommend forming a party via Partyfinder for these experiences."