Most of the negativity stems from disappointment, I think. When you realize that the Bethesda Fallout Games seem to have more in common with the universally hated "Burned Game" than F1 and F2, it's bound to cause disappointment in some people (Like myself).
But the "argumentation" is equally stupid on both sides. I've seen the "True Fan" argument from both the "Old Schoolers" and the "Newcomers". One side prefers classic RPG's, the other prefers Shooters. It's as simple as that, really. Well, That and which game was their introduction to the franchise. Nostalgia is a powerful thing.
The initial fallout fanbase were hardcore cRPG fans as that is the type of people fallout 1 and 2 appealed to. After Bethesda acquired the IP and made fallout 3 part of that fanbase stuck around while others decided to not play it as they did not like games with shooter mechanics in it and preferred combat that was more akin to strategy games rather than action games. Part of the initial fanbase that ignored fallout 3 did get lured in by the development team that had worked on fallout new vegas, but many ignored fnv as well due to its action based gameplay mechanics. fallout 3 and fallout new vegas brought on board new people who either did not play fallout 1 and fallout 2 or who tried them and could not enjoy them due to the gameplay mechanics. fallout 4 upped the action and reduced the rpg features in the game, so a portion of the fanbase (either from the initial fanbase or those who got on board with fallout 3 and/or fallout new vegas) who didn't mind action elements as long as some strong rpg elements remained decided to jump ship, and fallout 4 managed to bring on board people who did not play the earlier fallout games, or could not enjoy any of them due to their game mechanics.
so now we have a fanbase that consists of those from the initial fanbase who persevered through the transformation in game mechanics and narrative that fallout has endured over the years (especially with Bethesda continuously experimenting with fallout), those who got on board with fallout 3 or fallout new vegas and who decided not to jump ship with fallout 4, and those who got on board with fallout 4.
so here we are again at that point where choices have to be made, do we jump ship or continue on with fallout 76?
despite the changes fallout has gone through it has always remained a single player game revolving around rpg features and mechanics of varying degrees-from hard-core to light-as its central focal point.
this is not the case with fallout 76, and for many this change is something that, despite the title of fallout, will make this game undesirable for them, multiplayer is just too big a change for some to continue to stay on board through this game and yet again a portion of the fanbase (and my guess is it will be a larger portion than those instances before) will jump ship (myself included).
some people who see this as "hate" cannot seem to get their heads around the fact that the fallout title and some fallout themes is just not enough for every single fallout fan, just as part of the fanbase has jumped ship with each new iteration of fallout in the past, a part of the current fanbase is going to jump ship with this title as well. Whether Bethesda will be able to get at least a part of them back on board with fallout 5? time will tell.
so a large portion of the current fallout fanbase is left with a brand new fallout game that they are not going to be able to play mainly due to its multiplayer nature, this is not an easy thing for a fallout fan so some are going to question why Bethesda has decided to do this.