The thing that makes Dream SMP so horrific isn't the sheer scale of it's angst. There have been children's shows with a bigger death toll by thousands and even millions, like Steven Universe with it's themes of breaking free from an exploitative system through war that showcased suffering spanning universes wide, or Adventure time where it's entire basis of history is that their world is the remnants of an apocalypse, or as with Gravity Falls, where the whole fabric of reality would have been transformed to a nightmare if it hadn't been for the bonds between one rag tag family, etc. No, the pain in Dream SMP isn't grand, and that makes it all the more difficult to stomach.
Because instead of the main villain being some chaos entity from space that slaughtered numerous lives, it's that friend that acted like an older brother and laughed over the most stupid of jokes and fought with you over the most useless, mundane of things, before he turned around and started slipping further and further into his god complex until the only thing he lived for anymore was the enjoyment and fascination he took in your suffering.
And that authority figure you need to take down for their unethical management of the system is not some lackey or world leader who you only know by their title and evil deeds, no, it's that man that gave everyone pumpkin pie and promised you he could give you a home when everyone else had left you in the dirt and who told you how sad he was to see you frown and giggled over your valentines confession and created a whole sentient machine to help you combat your mental health struggles and build you a place where you could actually feel safe, only for him to fall to repeating the same torturous abuse on others that he swore he would protect them from.
Oh, and that otherworldly entity, beyond the veil of your mortal world, that wishes you to join them in their eternal game outside your reality, that knows the secrets of the universe, including it's end? What if that was once just your brother that played guitar, and your father that smothered you in affection, and your leader that tried to lead your close knit group to freedom and prosperity with diplomatic words, and stood as bait in the range of fire, unprotected, when it came to battle, and cried under closed doors when the speeches were done and all that was left was to lie face first into a pillow. Slowly, painfully, drifting away from his goal, from hope, as he let go of that vision and self destructed in his paranoia and pain, taking you all down with him. Dying with the conviction that the only thing he ever brought, and could bring, was suffering.
So it doesn't matter that the wars that they went through only featured a dozen people at most, because while there was no big, inconceivable number of the lives lost, the despair and utter loss of hope, and anger, grief, that the few people we follow displayed at the end of it, was enough to tell the story. And it doesn't matter that the exile arc only lasted a week in our time, because while it didn't last years for the character like it could have in any other story, the plain and so unapologetically explicit depiction of abuse we sat through watching all 7~ of those days was enough to leave us shaken to our core. And it doesn't matter that the possession esque plot in Ranboo's story is never actually fully displayed, never actually results in anything too grotesque or alien, just some property damage, because the horror in Ranboo's voice and his detailed monologues as realizations keep piling up about the following pain that has resulted from it, the fear and uncertainty of himself, his own mind, and all the implications, are enough to make us cry.
Because, in the end, there doesn't need to be some big, cinematic, world wide tragedy to make us believe this is serious, to make us scream at the unfairness of it all. There just needs to be the intimate, and horrifying realization, that in this story, the ones closest to us are the ones who can actually hurt us the most.