In other words, like I said, Blizzard dropped the ball and it wasn't that good of a game or very well received. Yes.
It was well-received with Wings of Liberty, because at least that was just you being a generic good guy, and Heart of the Swarm has some fans, but they really dropped the ball in Legacy of the Void, especially when they killed Zeratul and made the Khala evil, thereby killing the most important Protoss character AND the core of Protoss culture as a whole, which just made the Protoss into progressive humans without mouths. Gone is the mystique of this alien race that communed with each other through the Khala. They're all just mouthless humans now, not at all different from the people you played as in Wings of Liberty.
Imagine if in the next Warhammer 40K video game, the Emperor of Mankind dies, but the Imperium finds another Warp power source that replaces him easily, and that leads to them becoming progressives in the same vein as modern progressive leftists, all clamoring for unity and kindness and getting along, while casting away their gothic Medieval culture, traditions, and religion. That's what Legacy of the Void did. The Protoss went from bloodthirsty Templars of the Khala who'd glass a planet if they spotted a single Zergling on it, to space hippies who want to be in harmony with the rocks, trees, and birds, it's pathetic.
Warcraft 3 was the "sequel a la SC2".
Not really. WC3's story and gameplay made it great; it was basically the offspring of Brood War and Diablo 2, the most popular Blizzard games before it. It's not their fault that game became the base for DOTA, especially since DOTA was just some dude's custom map, not at all something made by Blizzard.
And SC2 apparently killed the Starcraft franchise all on it's own.
It was more of a slow death. The first two entries in SC2 were somewhat warmly-received even as the multiplayer was getting complaints up the ass. But the story was all building up to a grand finale, and LOTV didn't deliver. So people lost interest in SC as a franchise. At most the Nova expansion got some interest, but not as much as there was when WOL came out.
Starcraft 2 was like the perfect storm for killing a franchise. You had the cancellation of Starcraft: Ghost and the franchise floundering for years. Then you had complete abandonment of Starcraft: Brood War in the competitive scene because it was taking too much attention away from SC2. And the comparison between games always negatively reflected on SC2. So Blizzard decided to basically attack its most hardcore section of its SC fanbase, the ones who made SC2 even possible, essentially punishing them for loyalty.
So basically, they told the most loyal part of the Starcraft fanbase to get fucked in the ass with a rusty chainsaw. Which predictably caused said fans to walk away.
And then on top of that the single player campaign had one of the worst written stories that defied even the lowest expectations from fans. It was like Blizzard had forgotten how to tell a story and just borrowed the most standard sci-fi and fantasy tropes from other stories and put Starcraft characters into them.
There's just something about a lot of sequels to franchises that feel rather generic. As if they were all borrowing from the same barrel of sci-fi tropes. Halo Spartan Ops. SC2. Mass Effect Andromeda. It's like they were all watching the SyFy channel nonstop and just borrowed story ideas from there.
And the campaigns were thematically different from SC and BW in that you no longer play as a nameless faction leader that is in control over an army or unit. You are written out of the story and are now just some observer and the characters talk to each other but never to the player. So you feel detached as well. In SC and BW the briefings are literally aimed at the player directly to immerse you in the story.
They basically made YOU into Raynor/Kerrigan/Artanis. Which killed the immersion aspect especially since those characters may or may not have similar opinions and attitudes as the player. The player is no longer a part of the cast, but rather, just someone who steers a single character within the cast into pre-programmed results.
And the story itself was awful. Ancient aliens. Reapers from Mass Effect. Romance stories. Redeeming Kerrigan.
Those plot points could've worked if they had competent writers. They didn't. Especially near the end.
Redeeming Kerrigan would've worked better if it was like how Luke redeemed Vader; imagine if Raynor had to fight the bitch and try to remind her that there's still good inside of her, and she keeps trying to deny it. If it was handled with taste instead of ''PRESS X TO TURN KERRIGAN GOOD AGAIN'', it might've accomplished something. Hell, if they wanted to turn her into a savior, instead of copying Marvel's Phoenix/Jean Grey, they could've had her use her psychic strength to purge the Khala of Amon's evil, thereby freeing the Protoss race from Amon in a single blow and making her the savior of the galaxy. No more Amon using the Golden Fleet to glass planets left and right, he's gone from the physical world, all because of Kerrigan.