That's probably part of it. A big chunk of the aspects that I didn't like about it relative to Fallout 4 -- from killing off slow-mo/pause VATS, to not having a world that can change much, to limited-size "settlements", to limited moddability, to having immersion-breaking other players jetpacking around with not-in-theme names, to having limited story content -- come from the fact that they built it to be a multiplayer game.
But even so. I've seen some footage of the game at release, and it was pretty bad. And not just bugs, but the content...I mean, a Bethesda game not having human NPCs?
I will give them props for putting a lot of effort into fixing the game post-release, but I still feel that the thing shouldn't have shipped when it did. It simply wasn't ready when it went out the door.
Also, some of the fixes they did do that I think people did like -- like reducing the severity of the food/water/radstorm survival elements, which many players didn't like having to hassle with, or reducing the role of PvP, which a lot of the playerbase didn't like -- didn't result in game rebalancing. Like, the player shelters were clearly intended to be a significant element to deal with radstorms, but radstorms are essentially ignorable. Food was intended to play a bigger role, and there are features oriented towards things like reducing the rate of one's demand for it, but that was removed.
If you look at Fallout 4 or even moreso Skyrim, modders went through and rebalanced the game long after the release. I'm not saying that everyone who played those games got to enjoy those changes, but I think that they were good ones. Fallout 76 isn't really moddable in that way, so it's dependent on Bethesda's devs to do all that...and they didn't really do that.
There were no really memorable moments from the game, the way, I don't know, the battle for The Castle or the arrival of the Brotherhood of Steel's aircraft or some other moments in Fallout 4 really stuck with me. I guess to some extent that's part of just having to make a lot of the content something that you play over and over, but it still was kinda disappointing.
And I'm not demanding that they work for free. I bought all the DLC for Fallout 4 and Skyrim. I'd happily have bought something like the (excellent) DLC packs for earlier games in the Fallout series for Fallout 76. But, instead, they only sold mostly-aesthetic content in the Atom Store. Which, okay, great, if someone really wants to decorate their player camp and wants to pay for it could be appealing to someone. But they didn't create a route to pay for more story content, more maps or the like. They did create new free content, but that necessarily has a limited budget, and again, was kinda oriented around multiplayer (and didn't catch on much with me and didn't seem to be terribly popular with players on the fo76 subreddit, either).
There are some things that I did like about it, that I don't think it got credit for. The building mode performance was significantly-improved over 4. They toned down the "everything is dark and awful and evil and every person and company is twisted" aspect in 4, which I think was a big plus; there were plenty of people just trying to live their lives in difficult situations, which felt more like 1. I'm not absolutely rabid about the new areas, but the Mire looked nice by the standards of their engine, was a good use of their engine's godrays. They did a bunch of performance and stability work (that had to happen, given that one couldn't just "reload earlier saves" if something broke in a saved game a la the single player games).
I could have lived with Fallout 76 not being Fallout 5, but what I wished that they could have done was to keep selling single-player content in traditional DLC form. A lot of MUDs and similar games have a "remort" feature where one can start with a new character and earn some persistent rewards for doing so, so playing through story content multiple times is still fun. "New Game Plus", kinda. The online aspect for single-player content would just be to provide DRM, so that people wouldn't just go swipe all the stuff that they're selling in the Atom Store. And the stuff on offer in the Atom Store...ugh. If you look at the mods in Fallout 4, people created high-resolution texture packs, new companions, new story content, and they don't have anything like that for sale. You could have segregated anything that affected balance out of the multiplayer areas, had very solid single-player-only content. It might not have been Fallout 5, but I think that it could have done a much better job of making people who wanted that happier while still providing a multiplayer game for those who wanted a multiplayer game.