Diablo 3, while not a particularly good game, is an example of a good coop game. Grouping with others doesn't really add much to the overall experience of D3 because it stands on its own if you play by yourself (online, but still solo). All these MMO-lites are centered around grouping up. The games are shallow if you don't add in the 'fun with friends' crutch.
The thing is: Diablo 3 added Lore stuff and NPC/AI banter, which wasn't a thing in older Diablo games. This actually made D3 game even more friendly to be a SP-campaign experience than ever before - which are all the things you put into a game to appeal to gamers that like SP-experiences.
Plus, you know...there's also the fact that you can still complete the game on the PC no problem w/out ever dealing w/ other players period. I'd know, I've done it.
EDIT:
To add: there are non-narrative/non-story-driven games (that also have coop) that still stand on their own solo. See, e.g., Terraria. It's not a binary thing. It's how the developers start with their design concept. Do they say 'okay we want to make a shooter with character progression and questing,' or do they say 'okay, we need to make some MMO-lite, what systems do we need to add to keep people hooked?' Most AAA (and a lot of indie) developers/publishers are doing the latter these days. Especially outfits like Blizzard and Ubi.
Definitely.
A lot of dev's are shoehorning PvP stuff (like Destiny 2 did w/ their competitive MP) into mostly PvE games to try to get the MP-crowd also involved. Especially when companies like Activision, Blizzard, and EA are looking at turning their games into full-blown "Live experiences" and "Online-Only experiences" to try to curb piracy & cheating; and to try to sell more content later (DLC's, Season Passes, Annual Passes, Expansions, MTX's, Etc) - which is probably also why we don't see DICE anymore letting loose Mod Tools for their MP games anymore; and also why another reason that Fallout 76 is online-only and really doesn't have mod-tools/mod-support...yet (if it ever even will).
Let's also face it: Destiny 2's PvE is where it's at. If you're doing the PvP, you better own a good deal or all of the DLC's, Season Passes, Expansions, Annual Passes - if you even want to have a chance at taking down other players and/or doing well in the competitive MP section. B/c of this non-sense, PvP in Destiny 2 felt like a waste for me and I was better off w/ PvE, which was still purposely and obviously built to be "easy enough" to solo a lot of the main-stuff and side-stuff - which is exactly my point on why games that are online-only which can be solo'd should still be able to work offline, whether it's done at launch or much later when the game's old and player numbers have dwindled down to not much of anything.
EDIT:
It's nice to have game-content actually preserved and able to work offline at some point in time, in case I want to replay these games later or so I can finish-up these games that I never did finish.
It's nice that my old copy of Hellgate: London still works in offline-mode, you know?
EDIT 2:
Having 5000 terabytes of storage and wanting all of your games to be offline singleplayer is definitely a weird hang-up, especially for a PC gamer. There is even way to do those things and it's called buying cartridges or discs and being a console gamer.
Why do I want to deal w/ the inferiority of consoles, that often run games at 30fps and/or maybe some games at 60fps?
I enjoy having all this PC power behind me, able to say "Yep, I can run Yakuza 0 at 120fps w/ G-Sync" and do it no problem. Oh, and they recently removed Denuvo DRM/anti-tamper, too.
It's nice to run GR: Wildlands, Far Cry 5, and any other demanding games at 60fps instead of the often console-like 30fps w/ much better graphical fidelity, if possible.
Plus, I do have a BR burner and discs; DVD burner and discs; and SSD's, HDD's, and docks to handle those. If I need to back-up games I downloaded, that's what they are there for. I can always buy more, if need be. I'm not limited by a console-box, thanks to the PC's nature of allowing for expanding.