Says you. Lets list just a few mechanics to tell a story in a destructible world full of grievers:
A) Anywhere in the world you find journals, quest or books telling you anything you need to know. Even though you find such stuff at random, it is for example very easy in principle to make certain that you get these journals, quests or books in the proper sequence if necessary. Or that you get specific papers only when certain preconditions are true. As long as you can scavenge anything, you can follow the story.
B) Radio messages could be introduced into the game. As soon as you build a simple transistor receiver (1 electronics, 1 wood, 1 battery) you can listen to whatever is on the waves, story, quests, cries for help, messages from the duke. Radio messages could even tell you where to meet specific NPCs necessary for a quest and should a griever kill him before you are there, the radio tells you where to find his replacement. As long as NPCs can be spawned, a griever can at most delay you for a bit.
C) NPCs can spawn as effortlessly as zombies. Don't ask how, but again, there is an endless supply of NPCs, and eventually you will talk to one who will give you information, quests... As long as TFP make sure that those NPCs are replaceable, no griever can really stop you.
D) Story-relevant NPCs could be indestructible. Sure, the world is destructible, but the traders are not.
E) There is a meta-method: If TFP decides that the story is not griever-safe, they could make story-relevant NPCs killable and declare the problem solved. Either you play cooperatively together with friends or alone and you simply don't kill NPCs that have something important to say. Or you play on a server with grievers and that's your own fault then.
Couple of things:
Right now traders are indestructible, but that doesn't mean anything relevant for the future of the game as that might change later and doesn't guarantee that there will be indestructible NPC bases at all which would have to be in game to give a real story a chance. NPCs must live somewhere too, you know? And if you destroy their base, well that's kinda the point where any story that involves their base ends. That's one of the obvious limitations you get for mixing sp and mp together.
As for radio messages, hints, notes etc...
You would really love the "well received multiplayer" in the No Man's Sky if you think the same concept would work in 7 Days to die with quests. In case you haven't figured out, it was sarcasm. Multiplayer in No Man's Sky practically doesn't exist as the game only gives you hints of where other players were at one point in time of their gameplay and what they did there and that's it. No direct interaction with them, nothing like that. So getting only hints / quests from loot containers is practically nothing new, we already have that in 7 Days to die and this kind of gameplay gets lonely in solo when there's no real interaction with NPC survivors.
Even now when playing solo the loot containers respawn loot to simulate that someone was there and left something inside. Perhaps it was one of those thousands zombies that I killed last night? No? Then maybe it was that trader who lives nearby.. oh wait he never moves his ass outside of his base, so it couldn't be him! Then who? That's a million dollar question. Who in the whole lonely procedurally generated world was that if I'm the only freaking survivor who actually moves his ass around the world and I'm pretty sure it wasn't me who left some stuff in that loot container?
Should I feel guilty now for that I was always hoping that 7 Days to die will one day evolve into a game where even in singleplayer playing alone you will feel like the world is actually still alive to a certain extent and surviving the apocalypse? Sure there are thousands of zombies trying to eat you, but there should also be many NPC survivors and bandits who actually SHOULD DO something more than just standing in one place all the freaking day.
Among the other activities, they should give you quests directly by talking to you, NPC survivors should also help you on your journey like companions / followers known from Skyrim or Fallout. You should be able to take sides, heck you should be able to join NPC factions in their own bases and NPC survivors should help you take down other NPCs like raiders / bandits. In other words the game should give you the impression that in that giant procedurally generated open world filled with brain dead zombies, it's not just you and couple of traders who never really move their ass from their base!
7 Days to die is already a good game in its multiplayer aspect, but while singleplayer had a great potential too, it was simply ditched or limited to the same gameplay features already offered by multiplayer. Is that what it ever will be? Won't there be anything more than that for solo play? Well that's a big disappointment then. That's not enough for a good singleplayer experience. You might as well just remove it entirely. But coming to think about it, removing singleplayer option from the menu even before singleplayer portion of the game ever evolved to something bigger is actually already the same like removing it entirely.
Sure you can still play solo by "freaking setting the amount of players to 1" as Roland says, but that's not my point. Being alone in an empty giant world tailored strictly for multiplayer isn't the same thing as playing alone in the world filled with NPC survivors that keep you company to make sure you don't feel lonely playing alone!
That's why quests should be from "Faction + Title" and not a specific NPC. So, if you get a quest from: "The Ranger's", Leader NPC, if the current leader is killed then another takes his place.
What would be interesting is to have them all wiped out and that triggers another quest: "The Angel of Death" - Kill them all! ("Them" could be any number of NPC's) OR "The Rise of the Phoenix!" YOU can be the leader who: makes the Rangers great again!
See, MP is no problem to a robust quest system.
It is a problem for finishing safely specific quests you personally want to finish. And this has nothing to do with the possibility to set the game to 1 player, obviously the argument is that since it's now going to be just one single environment, developers won't be able to make quests tailored for solo play and therefore you can't expect actual singleplayer story mode.